Ultimately, if you're not under contract, and you don't plan on changing your carrier in the next 12 months, I think the iPhone 4S looks to be a great phone.
I'm extremely happy to see the introduction of Siri. It is, by far, the single biggest reason that I am upgrading. With the work I do, I am constantly in the need for setting up quick to-dos and notes. I was genuinely considering an Android phone because of this feature, but I did not want to go down that road.
The improved camera and processor are also big benefits IMO.
Am I disappointed in what was announced? In hindsight, not really. While I would have liked to see the introduction of LTE, it would only have impacted me in potentially leaving AT&T for Verizon (because it would finally provide for simultaneous voice and data on VZW). But the speed of LTE was not a selling point for me. With these garbage limited data plans on the carriers, what benefit is there to blowing out my data cap even faster? Plus LTE is a known battery hog. No one has tamed that issue yet, so really, what's the point? Bigger screen? Don't want it. NFC? It will take YEARS for widespread adoption of the technology.
All of these phones are incremental. I think the only one that was a big jump, relatively speaking was from the original iPhone (EDGE based) to iPhone 3G. So iPhone 5 will be a decent bump from the iPhone 4, but I just don't see it being an enormous game changer.
If Siri doesn't excite; if the better camera (low-light sensitivity, 1080p video) is just nice and nothing more; if mirroring your iPhone 4S to your TV via an Apple TV2 isn't interesting, then skip it. Upgrading to iOS5 should be a pretty nice bump. Otherwise, enjoy the new phone. Don't worry about what gets introduced next year. Use that attitude and you'll always be waiting for the next big thing.
EDIT: Something that I had forgotten, but needs to be pointed out.... I would be far less interested in the 4S if I were on Verizon and had an iPhone 4. At least with the AT&T iPhone 4S, we'll see a (potential) speed increase. Verizon users would have the exact same network speeds as the iPhone 4. As much as a get frustrated, along with everyone else, at the dropped calls on AT&T (which are not as bad as they used to be), I couldn't give up the simultaneous voice and date feature and faster overall speeds.