The AT&T 8525 (also known as the HTC Hermes, among others) is a PDA and phone, but oriented more to the former than latter.
First, voice quality is excellent. My first calls on it all had people commenting on the clarity before I even mentioned my new toy. The WM6 contact manager syncs with Outlook, which is handy for backups, but is difficult to navigate on the touchscreen; luckily, typing the first few characters of a name brings up their contact. I have no complaints about Calendar or Tasks, both of which are easy to use fast.
For those who haven't used Windows Mobile, it's similar in spirit to Windows on the PC in that it's not the most pleasureable to navigate, but it's a platform where you can find any app and it'll run well. This is mitigated somewhat by the 8525's many buttons, including a jog and click wheel for the start menu and the standard directional pad surrounded by buttons. Of course, this device also has a touchscreen, and if you have two hands free and time to carefully navigate, it's the best option.
The battery life leaves much to be desired. With moderate use (a couple calls and adding events to the calendar), the battery will be mostly dead by the end of the day. Aggressive power management options are available if you don't find the device turning off every time you look away annoying.
For those who like to tweak their devices, there's an active community at xda-developers.com.
Overall, it's a fine device, but it definitely won't be the last phone I'll own.