I knuckled under and got an iPhone 4S sooner than I expected, largely because the battery on my old Treo 700p coincidentally started to have problems holding a charge a couple of weeks ago.
So far, I'm quite happy with it. The call quality is at least as good as the Treo (still my main reason for having a phone), and I am pleasantly surprised with how well the Siri voice function has been working for me.
On the negative side, I really do miss having a physical keyboard, and some of the things that I used to do routinely are so firmly entrenched in my muscle memory that it is a bit of a chore to adjust to a new way of doing things. I do have to recharge it every day, which is not something I had to do with my Treo, but since I leave the iPhone's Wifi and Bluetooth turned on, I guess that isn't too big of a surprise. Overall, though, I think it was a good purchase.
A final note: I went to a retail store to make the purchase, and I made a big deal about being on a tight budget and that I could not spend any more per month than I was already paying. The sales person made a few adjustments on my service plan, and it looks like I will actually end up spending $15 less per month than I had been. Quite a nice surprise!