I'm gonna stick my neck out here...
It seems to me you are a Mac user. You chose your Mac for a reason - apparently it has the power, flexibility, stability (usually) and looks that suit your needs. Make the same choice in your phone. Choose the one that hits as many spots as possible, with a user-support base that can back you up or lead you to the most stable solutions (isn't that why you're here?)
I happen to recommend the Treo. I've been using Palm OS for the past 5 years, since Apple pulled the plug on the Grandaddy of 'em all - the Newton. I currently use a Treo 650. I also use Macs, except at work where I'm a slave to the darkside (at least my Treo and my MacBook Pro keep me sane).
Simply put, I enjoy having accurate and stable synchronization with my Macs, albeit through the 3rd Party solution (Missing Sync), regularly and succesfully synching with Address Book and iCal. I also have a huge stockpile of other Palm OS applications which allow me to simultaneously IM via AIM and Yahoo, email with attachments, read and edit Word and Excel attachments, read and notate 2 versions of the complete Bible, keep a history record of calls, meetings and todos for each contact, records and edit audio, securely store passwords, etc., play mp3s and games galore, then it backs itself up automatically every night onto my SD card. I transfer files back and forth to my Macs and perform hotsyncs via Bluetooth, which by the way works great with my headset. Web browsing is fast and easy to read with clear graphics. Oh yeah, it takes usable pictures. I'm talking daily use here, folks.
I've heard nothing but horror stories about Windoze Mobile devices so I won't even go there. If you're looking at the Treo 700w, I can't help you. And if you're planning to connect to a PC, well you've got enough problems without my input.
As a Mac user for the past 10+ years, I've been amazed at how often PC users have told me they stick with Windows systems because that's what they use at work. They figure, "if my company bought dozens of them, they must be the best." That doesn't work for me. Reading this thread I'm reminded of those conversations.
To be honest, I've tried to implement Blackberrys in my workplace on two separate occasions. I personally didn't find them to be as useful *for me* as my Treos. My staff gave them back within a week and asked for Razrs. I didn't think they were quite that bad, but I couldn't find a way to show users that the Blackberrys would be easier to use or more useful. Unfortunately, our phone carrier doesn't support Treos.
Many other Treo/Palm users I've known almost never experience crashes. Mine crashes more often that theirs, and I honestly don't average even 1 crash per month. You see, all they use theirs for is phone calls, contacts and appointments. I push my Treo hard, as you can see by my partial list of applications above, yet the only time it locks up or resets is when I experiment with some new unstable application. Here's the 3 minute solution: I do a quick and perfect restore from my SD card backup and get right back to work.
Check out the Treo communities at
http://mytreo.net and
http://www.treocentral.com. I'm sure there are some Blackberry communities out there with people who can give a strong perspective well beyond my experience.
To me, the Blackberry is an internet appliance. My Treo is an alternative computer in my pocket that allows people to call me when I'd rather be making music on my Mac 🙂
iMac G5 2.0ghz + Macbook Pro 1.83ghz Mac OS X (10.4.7) 
