Well, I feel cwatz pretty much hit the nail on the head, but I will say this: champions are always getting rebalanced, and as new champs with new mechanics are put out, the relative strength of every other champ is always changing. Characters that were once very powerful can and sometimes do get nerfed pretty hard (Twitch). Champions that have been collecting dust for months can get a huge buff and get popular as all hell (Kayle recently). Play someone you like, and don't worry too much about the future.
That said, Jax is pretty awesome at the moment, so you won't be catching any complaints for a while about your champ choice.