It gives you a bit of everything without getting a lot of something. Here's my thoughts:
Fireball
Magic Missile
A decent magic missile and easy enough to cast, able to to from d6 to 3d5 Str 4 hits. can be very useful against smaller units like fast cavalry or flyers. Range on it is good enough that you don't really need to channel it through skinks.
The only way you'll get anything out of the Lore attribute would be from taking the Ruby Ring of Ruin and using it to shoot a fireball at the target first. +D3 isn't game breaking, but if you want to cast the 18+ version it can be good.
It's also flaming, so that can be or bad.
Wyssan's Wildform
Augment
An awesome spell that's always good. Pretty much everything but swarms and skinks with blowpipes is a good target for this spell. Getting some skink priest with it as well to try and stack it on a single unit is also good, and the slann has a much, much easier time casting this.
There's plenty of things in the Lizardmen army that benefit from the lore attribute, though -1 to the casting spell isn't that critical for the Slann.
Searing Doom
Magic Missile
Similar to fireball but can only get up to 2d6 hits and has double the casting value.
The real difference is the lore attribute, which ignores armour and is easier to wound the better the enemies armour is. This is essentially an answer to enemy heavy calvary.
The benifit of having only the signature spell from Lore of Metal is that this Lore can be at times hit and miss. Against some armies there's unlikely to be more then one unit with an armour save better then 4+, and they probably have a ward save or maybe MR. In these cases the damage potential of the Lore of Metal is really decreased. But against armies heavy on the armour or that have a couple of really killy, heavy armour units then this lore is a power house.
Basically with all the sig spells you get Fireball to take out the light armour units easily, and Searing Doom to take out the heavy armour units easily. That's some good utility.
Like Fireball it's also flaming, so that can be or bad.
Shem's Burning Gaze
(Another) magic missile
Shem's has two level, the first is exactly the same as fireball in hits, strength and casting value. The second level doesn't increase the number of hits, but ups the strength from 4 to 6 at a casting value of 15+. Basically the second version is best used on single, higher toughness models like monsters, or maybe on particularly tough monstrous infantry. Apart from that, it's another easy to shoot missile good at taking out light units, just like fireball.
The Lore attribute comes into play against very specific enemies, but against them it gains and extra d6 hits. This is generally great against these armies, as they usually can field both small elite units and large blocks of cheap, light infantry. This spell has a version suited for both of these.
Again, it's flaming. Regenerating enemies aren't going to be a problem for your magic phase, but anything that gets extra saves against flaming attacks (and there's a few things out there) might be a problem.
Earth Blood
Augment
Gives the wizard and any unit he's with a 5+ regen. Useless for a solo Slann, but rather good in a TG bunker provided the enemy doesn't have plenty of flaming attacks. The casting value is easy for a Slann too.
The lore attribute can be reason enough to cast this spell, letting you heal a wound on a friendly model in 12". Good for maybe getting some health back onto those monsters, great for putting a scar-vet back to full health.
IceShard Blizzard
Hex
Ever since one of the first 8th edition errattas that changed the wording of steadfast, this has been one of my favorite spells in the game.
Easy to cast for any wizard, it provides a very useful -1 on the enemies hit rolls (shooting that doesn't roll to hit will only get to fire on a 4+) and also to their leadership. Useful on nearly anything that can shoot or is about to charge a unit. One of the main weaknesses of Lizardmen is a lack of high WS on our warriors. Bringing elite enemies models down to hitting on our level (often 4+) really helps cover this weakness up. The -1 to leadership isn't so good on it's own (especially when the enemy has a BSB nearby) but like Wyssan's Wildform this is another spell that's easy to stack due to skink priests having access to it. I once managed to make a Brettonian unit run off the board with their general and BSB on turn one as my opponent allowed me to cast this spell onto the unit 3 times. A single wound from a salamander in the following shooting phase saw them take a panic check at -3 leadership and flee.
Also, remember that steadfast and stubborn units only ignore combat resolution when taking their break tests. The negative leadership from this spell still applies so not only does it help you win combat, it helps make enough the most stubborn units break and flee.
The lore attribute isn't bad, but often it's not worth casting this spell onto a flying unit just for the free d6 str 4 hits when the -1 to hit and Ld are more powerful elsewhere. But, if there are no greater threats around, go ahead and cast it. It's basically another fireball.
Melkoth's Mystifing Miasma
Hex
Ok, very easy to case and has a ton of uses as it affects so many stats. Negative movement is always powerful as per the FAQ, even flying movement is affected which can really, really screw over a flyers ability to position itself. Same applies to fast cavalry and this can be enough reason to cast the spell itself.
Reducing ballistic skill is powerful, as each point lost is essentially -1 to hit with shooting (provided the unit shooting cares about it's BS)
Now, WS and I are more iffy. Why? Because generally our models have a such bad WS and I that lowering these values might not even make a difference. Against high WS and I armies like elves and warriors of chaos, you might not get any benefit from casting this spell, especially as the value you decrease the stats by is random. If you want to get hit less in combat, Ice Shard is much more effective.
Having said that, against models that have similar stats to our warriors, lowering the WS and I is amazing. Forcing things to attack at the same time as saurus is great. Having the saurus hit on 3's is brilliant!
So it can be situational, but never to the extent that Searing Doom is. Still a great spell.
The lore attribute won't get used much, but it can be a Slann saver. A solo slann will use it the moment it looks like he might see combat, while a slann inside a TG block can use it too escape should all the TG be about to disappear.
Spirit Leech
Direct Damage
The only direct damage signature spell! Yay, something you can use while your TG are in combat! It does have a short range though, and you can't cast it into combat, so remember that.
Now, this is a great snipe spell (especially channeling through skinks) but there is one problem. This spell uses the unmodified leadership on the caster and the target. For us, that's a 9. Nothing in the Lizardmen army gets an unmodified Ld higher then 9, and as this spell isn't a leadership test we get no benefit from Cold Blooded.
"So what?" you say, "I'll use it on models with a lower Ld." Unfortunately inspiring presence works for this spell, even though it says unmodified leadership (check the FAQs). So for a lot of enemies they'll have a Ld 10 general about, meaning they've basically got a +1 to the dice roll.
Now, bad rolls can happen and you might roll a 6 to the enemies 1, doing 4 wounds that they can't use armour saves against. But the odds are stacked against you a bit.
Now, Inspiring presence only covers so much area, often leaving the flanks uncovered. This isn't always that helpful as this spell can only wound a single model and most of the targets you want to go for with be in the general's leadership bubble.
Having said all that, this is a great monster killer spell. Any monster that gets out of the generals Ld range should be targeted as they often have low Ld, giving you a good chance to kill it off in a single spell. Sometimes you won't have the chance to do this until later in the game, as army will often deploy tight enough to project it's expensive monsters (especially once they realize you got this spell).
The range is short to ok, 24" on the boosted version but as the casting value is only 3 higher is makes little difference. You can channel this through skinks too, so that can give you a good long range to hit anything that strays away from their general.
The lore attribute is pretty good, a 1 in 3 chance to get a power dice for each wound you do. Often this is only going to come into affect against the low Ld monsters you can get, but considering you could get 5-6 wounds on a single casting (and this spell is just really good at killing those kind of monsters) throwing a few dice to make sure this spell goes off can be worth it as on average you'll get a few power dice back. (just remember that on a miscast, you resolve it after the spell so any power dice you gain might be lost from the miscast)
Summary
3 magic missiles, all of them flaming (can be good or bad), two of which can be ok to fantastic.
2 hex, both of which will be useful every game.
2 Augment, one which is great, one which is fantastic.
1 Direct Damage, situational.
In a magic phase with a high number of power dice you can get a lot of spells, which will make it really hard for the enemy to decided which spells to stop. The trade off here is really the lack of large game changing spells. But you do have 4-6 spells that will be used all game, every game, and over the course of 6 turns will help you win.