My wife now has a 4S, but I can't upgrade for a subsidized price until Spring, so I'm stuck with the 4.
My advice for those in similar situations: avoid, avoid playing with the 4S! It may not seem vastly faster when you're using it, but go back to the 4 and you'll see the difference.
Dang. I love my 4, but now it feels like an antique!
Sure it's faster but really think about it. Is the performance so incredibly better that it's worth spending hundreds of dollars more, plus extended your carrier contract for another 2 years, instead of your existing iPhone 4? It's easy to be dazzled by Apple's marketing machine. But when even the biggest Apple fan boys in the tech press are advising iPhone 4 users to think VERY carefully before upgrading to the latest model, you know the differences between the two models is not that impressive.
I'm pretty sure you'll feel a bit different in a few weeks when the newness of the iPhone 4S has worn off. And you'll definitely regret it when the iPhone 5 comes out next year and you're stuck on a relatively new carrier contract for a fairly insignificant upgrade.