I had a 650 (well still have it, needs to go on eBay), and like everyone else on this thread, I'm not going back.
I ultimately got tired of spending hours experimenting with combinations of apps and rom tools to get rid of the frequent crashes but retain the functionality I wanted. With an 8GB SD card, PocketTunes, and a headphone adapter I managed to get similar functionality as the iPhone (plus great battery life) at the cost of a few reboots a day. With the iPhone, things "just work".
I purchased an iPhone after a two month bender of trying various phones (thank you for your generous return policy, AT&T). I kept going back to the Treo after using these alternatives for a while. I first gave the Blackjack a shot, but the limited WM Smartphone apps and janky Bluetooth weren't acceptable. Surprisingly, I found the battery life to be fine once I disabled 3G. I had the N75 for a weekend, and its battery life was an issue, combined with a horrible email client and no suitable way to listen to music (no A2DP, only one headset is available for the awkward Pop Port) made me return it quickly. My last experiment was the 8525, which matched the Treo in bulk but had shoddy battery life and required a lot of fiddling to get the settings just right. I also realized at this point that I wasn't getting along with Windows Mobile, mostly due to the lack of detail (no profiles on PocketPC, muting the audio only mutes some sounds, "notifications" are as annoying as modals on Windows, programs don't actually ever quit, and it's SLOW!), all of which were implemented logically on the Treo.
For my needs, the iPhone apps hold up. The email is fine, though I wouldn't mind IMAP IDLE (like Chatter) or better attachment handling. The PIM apps are a little weak. Although the contacts and calendar are excellent, the notes app doesn't sync with Outlook and there's no To-Do app. I didn't use those too much on the Treo, so it's not a big deal for me. The music app is brilliant, even if you ignore the eye candy that is Coverflow. Actually, I find Coverflow annoying as I leave my phone on its side in the car (horizontal orientation activates Coverflow) and the music control buttons are only available in vertical orientation.
The crown jewel is the browser. Anyone who has used more than the pages on the palm.com portal knows that Blazer is very limited. Safari is so much faster at rendering and actually renders tables and deals with Javascript properly. And of course, an errant page that causes it to crash (which shows up less often than in Blazer) will drop you to the home screen instead of forcing a reboot. I wouldn't mind if Flash support was added, though most sites aren't Flash-heavy.
I do miss the physical keys a little. I have found that my typing speed is a draw between the two, and even better on the iPhone when I'm not trying to be so accurate with my keypresses and let the autocorrect do its thing. If the iPhone had some sort of tactile feedback (twitch the vibe motor when a key registers maybe?) I'm sure I could be even more efficient.
As for battery, the 650 was a workhorse, and the iPhone matches it. So far today I've logged about ten minutes of calls, half an hour of browsing on WiFi, and about four hours of music. The battery gauge still reads over 3/4. While some are up in arms over the non-replaceable battery, I'd like to point out that I've had my Treo's original battery for over two years, and it still holds a charge like its from day 1. I charged my Treo nightly and used it moderately during the day, checking mail constantly and using it for a couple of hours of music or Audible, plus a few calls. At a couple of points in its life, I had to go through the "Battery Reset" song and dance to make the charge meter behave, though I've never had it overreport the amount of battery left. I suspect that the iPhone will hold up much the same way, and that I will be out of contract and probably looking for its replacement when it starts to hold less than the rated charge.
I'm still hoping that Apple will release a real SDK, rather than rely on web apps and the work of hackers to add new functionality. There is so much potential in this little machine, and as recent events have shown there's no shortage of developers willing to extend it.
