Treekin are great, if a little overcosted. They have a long list of advantages that Cold-Blooded, Strength 7 (or even Strength 6, for that matter) don't make up for.
Ulodwor said:
They share with the Carnosaur the unfortunate trait of paying dearly for damage output while lacking the initiative or resilience to support it.
100% this. For some reason, no doubt to create balance (that they then ignore as and when it pleases them), GW decided that all Lizardmen must have bad Weapon Skill, even where it doesn't make sense. This isn't necessarilly a bad thing if you can account for individual performance through either:
a) Improve the odds for sheer number of attacks
b) Have the resiliency to inflict damage over time.
Sometimes you can do both - Saurus Warriors, for instance, are fairly cheap enough to bring tons of them so you benefit from both a) (doing lots of attacks) and b) (having the numbers to stick around to keep doing lots of attacks). Trolls are another good example.
Sadly, we are at the point in the game where mere T4 4+ armour save just isn't good enough anymore. There's far too much high strength Monstrous Cavalry, Monstrous Infantry, lone characters, or horded up S4+ infantry out there, and most of these things are just as good recieving a charge as they are dealing one (unlike, for instance, Knights). Kroxigor simply lack the resliency to stick around to keep pumping out attacks against anything that you're going to want to put them in to. Against anything that they'll reliably do well against, there are either plenty of other tools available in the book that do it much more cost-effectively.
Secondly, since GW decided that Kroxigor are so good, they have been priced as such to mean that you can't bring them in numbers. Go look at the price of other Monstrous Infantry. Kroxigor sit at the upper price bracket along with other such notoriously overpriced and underused MI as Minotaurs, Ushabti and Treekin (who have the sole distinction of being overpriced but not underused, as Wood Elf players don't have much choice). Now go look at a Chaos Ogre with Mark of Nurgle and a Great Weapon. Guess whether I'd rather have 4+ and S7, or 5+ and S6 with impact hits and -1 to hit for less.
Worse, is that Vetock had the genius idea to overload Special with practically everything you need to make the army work. So even if you do fork out the 300-400 points for a reliably large enough unit of 6 or 8, you're significantly hampering yourself in what else is available.
You could put them in Core, of course. Though I don't know why you'd want to. The Kroxigor can be targeted by anything that they might want to kill, and because they are in a Skink unit there's less of them and fewer able to attack. Every model lost significantly reduces the damage output and overall efficiency. And the Skinks then get stomped. Or you could put them in to things which can't kill the Kroxigor... and then watch the Skinks die in drove whilst, you guessed it, the low number of Kroxigor attacks are unable to swing the combat.
TL;DR - no Kroxigor love. Sorry

Cool looking models go sit on the shelf for another edition. Let's hope that 9th brings back Cavalry striking first on a charge, because catching such a unit on the charge is about the only viable use for them that I can see, and you don't see much cavalry these days when almost everyone gets Monstrous Crutches.
Edit: Okay, maybe you could consider 3 of them as a flanking unit to give your Saurus blocks some punch. But you really would need to go into the flank, and even then you may need to be careful what you are flanking. And for the same cost, I'm not sure that a Dragonhelm/Dawnstone/Cold One/Great Weapon Scar-Veteran wouldn't be better.