The 4s was a big bump from the 4 in every way except appearance. The 5 seems like a bump of similar size but including appearance.
I tend to agree with this post.
I didn't plan to update to the 4s (I've generally avoided the 's' phones), but my 4 died a premature death back in the Spring and I decided to get the 4s. It's noticeably faster than the 4 in real world use.
Obviously, I have no idea how the iPhone 5 will be in actual use, but if the performance increase at least matches the difference between the 4 and 4s, it will be a significant improvement.
I know lots of people are complaining that Apple didn't increase the width of the phone, but I, for one, am glad! I like the fact that the current form factor is easy to use one handed AND that it fits in my pocket without being bulky.