My experience is thus: When I was younger, lithium worked wonderfully, especially in combination with effexor. When I went into my forties, I suddenly started to metabolize it differently and started to have all sorts of side effects; cognitive slowing, fast and excessive weight gain. I was switched to topamac and although it isn't as fantastic, it works well and the weight gain has reversed.
For a while I was tried on valproate and I wasn't a happy camper. I threw up more or less constantly, lost hair and had other side effects, and they were never able to get me up to the therapeutic threshold.
I am told that the lithium works best on those who tend to have a family history of bipolar illness, but I don't know. We have a fmaily history of general wierdness, but it's hard to trace further than a couple of generations, since folks don't talk about it.
My mother was on a drug related to valproate for migraines and had similar problems.