I started taking lithium 25 years ago when I had cyclothymia. It didn't keep me from having manias eventually. I had a few times off of it to try Depakote and Tegretol. Both of those had more side effects for me in terms of feeling sedated or foggy and didn't seem to help me as much.
My impression is that lithium causes me to forget some mood that would otherwise persist. I can't tell that it restricts any moods I get in response to things that happen, but somehow those moods don't persist as much as they did at other times. However it works, I'm confident that I do better taking it.
The most obvious side effect is tremor. I had an essential tremor anyway. Lithium increased that so anyone who meets me notices the tremor. Other meds like Wellbutrin can make it worse, too. Someone thought once I must be on insulin and hypoglycemic because of the shaking. Someone else tried to push colonic therapy on me because of the tremor. I've wondered how many people think I'm withdrawing from alcohol - no one actually says that. I've gotten used to saying that I have a tremor like Katherine Hepburn's and leave it at that. I don't find it to be a problem myself.
Lithium causes weight gain both directly like a lot of psych meds do and because it interferes with one's thyroid. I need to be on thyroid to balance that, but that's a very simple thing to do given the benefit I get from lithium.
I like food so much. I don't know how much of that is psychiatric, how much is from meds, and how much is just being human. I gained and lost weight until I finally just started writing down every calorie I ate and followed limits about that as well as keeping track of fat, protein, carbs, and fiber the way dietary guidelines say to. That works, wherever my appetite comes from. It's taught me that I have to accept some hunger to stay at a healthy weight. That's true for me no matter what meds I take.
Lithium can drift into life-threatening toxicity easily with dehydration or certain other conditions. Other drugs are safer in that respect. But for me it's been an easy and effective drug, so I'm happy to keep taking it.