i've tried shadow, metal and life with my slaan.
Shadow against orcs, the hexs allowed me to make azhag slow enough that my oldblood went first and killed him outright. ok the wyvern saw to my oldblood, but you get the idea.
Metal i used against dwarfs, WoC and brets. obvious really but its tailoring your list and i like to keep my list the same so no matter who the opponant, i know what i can and can't do. Thats why i tend to stick with life always these days. At my GW it annoys the hell out of us regulars when you see people turning up saying "i want a game?", we say "ok", they then say "great what army do you have?". us telling them, THEN they go and write a list based on the opponent. Takes ages and takes a bit of the fun outta the whole thing.
I'm sure people will disagree but i think you should pick your list based on what you like and your style of play. You'll enjoy it more and get more out of it. Its not cheating exactly but its just not right IMHO.
I take life because its great for keeping my lizzies alive/regrowing and dwellers is probably my favourite offensive spell out there.
Yours Aye
Mixer