I have been on Lithium for three years for bipolar disorder. I have not had a manic episode since, and have been really stable. I personally can't see many side effects to Lithium. I mean I don't have any sedating side effects, tremors or anything like that.
When I first started Lithium I did gain some weight, but it could of been another drug that I was also taking at the same time. However, I did lose most of the weight that I initially gained. Blood levels do need to be checked every so often to make sure the Lithium levels are in range. They also check to make sure your kidney is functioning properly because lithium is filtered through the kidneys.