I've taken Death before and it worked out brilliantly. Death is my personal favourate lore for the simple fact that they will want to put their best close combat characters into your temple guard ASAP to kill the slaan because once the slaan is dead then you've practically lost due to the points involved with him being the general and 480 or so points on average after upgrades.
The enemy move in with their close combat heavy unit with their heros and lords and thats when you use the slaan to great benefit, IF you do not get charged then you can easily get off a few death spells that will snuff the average hero and hurt a lord before combat arrives and the best thing about the slaan is that once you are in combat, aslong as you are not in base contact with the enemy you can still use direct damage and missile spells.
The problem I find with taking Death is that the rest of your army suffers as a response. Granted saurus don't tend to need backup or templeguard because of their toughness and strength, but that being said if you choose a skink army then you're seriously at a disadvantage because skink armies need some sort of magical backup to prevent them from being rolled (in my opinion).
While you have the ability to gib just about any character you find within 24" or 12" with a Death Slaan you get the ability with a Light slaan or a life slaan to make one of your units near enough indestructable. Always striking first, I10, WS10 temple guard/saurus is insane or T8, 4+ regen is also kickass and it can be shared around the entire army instead of just the slaan.
I suggest that if you haven't used a death slaan yet then you should do it just once to see the difference, you will lose some units but to be honest you're going to lose a few units anyway, thats just the way the cookie crumbles.
That being said, a death slaan list would be for me:
Slaan
Focus of Mystery
Focus of Rumination
Becalming Cogitation
Cupped Hands of the Old ones
Bane Head
Ironcurse Icon
Battle Standard Bearer
- Standard of Discipline
This setup is meant to be the standard disciplines for a slaan, you get magical uberness but cannot fall back on Life when it comes to miscasts so cupped hands is a decent item to have, especially in combination with bane head. If you find someone who you don't like who is also a wizard then you're in for a fun game, just keep throwing 6 dice at somthing til a miscast happens.
I normally make my slaan a BSB simply because my army will be based around it, so it is logical, and out of all the characters it is the most survivable due to the natural ward save, which can be made better through magic items. This makes your unit of temple guard stubborn ld10 with cold blooded. You are not running away. The Ironcurse Icon I just put on there because it is a great cheap item to put into a unit you want to stay alive when you don't have life to fall back on for the regen.
A life slaan list for me:
Slaan
Focus of Mystery
Focus of Rumination
Higher state of consciousness
Unfathomable Presence
Divine Plaque of Protection
Book of Ashur
BsB: Sun Standard of Chotec
You could swap out the Book of Ashur for somthing else but it is a nice item.
Now this slaan is expensive but you have to take into account that your army is now based around whether or not your Slaan can dish out the life spells and stay alive. The slaan will have a standard ward that is easily made better from the use of magical items or the unfathomable presence. (I realise that hes only capable of being hurt via magic but some ranged items are counted as magic but don't give the magic resistance ability as a reponse.)
You don't need to worry about miscasting because you'll get throne of vines down most turns if it isn't still in play from the last turn.
The enemy will realise hes dealing with a Life based slaan and will try to either out-magic the slaan or kill the slaan to give him the win.
While i'm here can someone tell me why the hell whenever I type a large reply into this damned thread box it keeps scrolling back up to the top everytime I press... ANYTHING? It isn't scroll lock and this isn't the first time it has happened to me, its annoying as hell. Had to type this reply in word and paste it in lol..