I've got the Cingular 8125 (which is the Cingular branded HTC Wizard, which you can find from different providers)... I use it for Corporate Exchange push email, contacts, and calendering via Active Sync. I like the capability of it with 2 complaints:
- It's slow to respond to button pushes, like making, and hanging up calls (frustrating)
- Battery life is pretty weak, espescially when I overclock the CPU to avoid the first issue.
I've had a Treo 600, and LOVED it (before the company changed). I wanted a 650, and now want a 700p... I used to consider myself a Palm die-hard.
Besides the form-factor of the device, the real difference on the phone is really the OS (Windows Mobile vs. PalmOS). IMO, PalmOS is more usable with less clicks and stylus use to do common tasks. Windows Mobile is a bit more cumbersome with multiple clicks to do stuff.
That being said, I've been VERY happy with the Windows Mobile version of IE compared to browsers on the Palm. More native display of pages without having to purchase a heavyweight Opera. The Outlook client is pretty good as well, and the integrated calendering works well (as long as you don't want to dial a phone number from within an appointment!!).
When I had the Treo, it seemed like everything was an add-on. You had to buy Documents To Go to read Word and Excel files (read-only). The email client was pretty basic, and you had to buy a better version (like SnapperMail). All the free IM clients were crap, and the good ones were expensive. This may have changed lately, so I advise you check what comes out of the box, or as a trial vs what is 'built-in'. The GPRS modem integration with a laptop on the Palm was disabled and required a somewhat expensive program on the device and PC to enable it.
On the Windows Mobile device, all the Office productivity software is there already, as well as MSN mobile. You can edit Office docs natively too (though I'm not sure why you'd want to do that given that if you are in that position, you probably have a laptop too). The Bluetooth and USB GPRS/Edge/whatever modem on the Windows Mobile device just works.
In any case, I like my Windows Mobile device as long as I'm not getting frustrated and start mashing buttons, coz it just goes from slow to slower when that happens, which makes me more frustrated.
If I get the chance, I'll definately look into the Palm 700 series, and make a determination on the OS (700w vs 700p) depending on what comes out of the box... mostly because the Windows Mobile OS isn't as bad as I thought it would be (or I'm getting soft), and has pleasantly suprised me.
...and all that being said, I've heard and read VERY good things about the Blackjack, but have no personal experience to comment one way or the other... though the lack of the touch screen makes me a little skeptical.