I posted this on the Treo Central forums, but I figured it is relevant to this thread as well.
I had a Treo 680, and now I have an iPhone 3G.
The Treo 680 was my first smartphone. I migrated from a Zire 72 to the Treo, and used it as a combination device (PDA/Phone). The datebook, contacts, memos (and voice memos), and to do list were great on the Treo. I loved the keyboard on it, and the third party software. I would also check my email.
That said, I never used the Treo for mp3's or surfing the internet. Webpages didn't show up well on Blazer, and the connection was too slow to "really" surf the web. For MP3's, it was too much of a hassle to choose which songs to put on a memory card (since I only had up to 4GB memory card). I would much prefer to have my full library of songs to choose from at any given time. Video was okay, nothing great (certainly not any streaming or internet based video).
And of course the third party programs are what had kept me with Palm. If the Treo didn't do it, someone wrote some software to do it.
Now I have an iPhone. The date book is functional, without as many bells and whistles at the Treo one. You can have appointments, recurring appointments, with reminders and alarms. That's all I use on the date book. The contacts is pretty much the same. Memos are okay, but as someone said, they don't sync. Missing a to do list was key, but there is free software to do it on the iPhone now. They keypad takes some getting used to, and I doubt I will ever be as fast as I was on the Treo, but the speed of typing is certainly acceptable. Email is much better - you actually feel like you ar reading the email on your computer (not something that was cut and pasted into a .txt document). Internet is incredible - it is fast, and it feels like the "real internet" as the commercial says. And, since I have a 16 GB model, I can have my entire CD collection on there as well. And with the App store, hopefully there will be a lot of 3rd party solutions to things that are missing.
In my opinion, the Treo was a PDA/smartphone that also included some internet and multimedia fuctions. The iPhone is a multimedia/smartphone that also includes some PDA functions. Either device will do most of what the other will, but depending on your needs, you can choose the device that is best for you. If you MOSTLY want a PDA type phone, for the organization and PDA functions, but you still want some multimedia and internet, go with a Treo. If you MOSTLY want a multimedia/internet type phone, but you still want some of the PDA type functions, go with an iPhone.
Is the iPhone a better organization tool than the Treo? No, the Treo was better IMO. But will the iPhone still do most of what the Treo did - yes, just not as well. Is the Treo a better multimedia/internet tool than the iPhone? No, the iPhone is better. But will the Treo still do most of what the iPhone does - yes, just not as well.
And one other thing, the iPhone is a better gaming device that the Treo as well.
One thing to consider with 3rd party applications is that Palm applications have been around for YEARS. The iPhone has only been around for a year. in another year, imagine what the iPhone applications will be like!!