Hmm, I would probably upgrade if I had 4, but maybe not if I had the 4S.
iPhone 4S to iPhone 5 - A definite difference, but may not be worth upgrading.
iPhone 4 to iPhone 5 - A huge difference, I'd most likely upgrade.
You just have to find a friend or relative in RL who has one and check it out. Or go to the store, be an asshole and use it for an hour. :thumbup:
Edit: You can also sync wirelessly through your home network. Not sure if that feature is on the 4S, but it's awesome.