They are very limiting.  They cannot hold near the number of contacts that other devices hold.  The new blackberrys have a phone built in however, this can be a problem.  They will blow thru the battery very quickly.  Our whole company changed from the Blcakberry to the Treo 600 last year.

The Treo 600 will hold all your contact info (I currently have 16,000 names for the company)and it receives e-mail like a blackberry.  It has a phone build in and the battery lasts a little longer.  If you use a system like Outlook, you can have the device sync with your Outlook so all your e-mails, contacts, and appointments are automatically updated wirelessly.  The wireless function is available with the Sprint Business Connect service, but other companies might offer the same.

The Treo works well and I am sure there are other that will do the same.  Just research well before you purchase.

This one is the bee's knees..........

Palm OS, cell phone on steroids, web browser, and a keyboard to boot.

And it will fit in those little scout ship purses you ladies like to carry that can hold a phone and a lipstick and that's it......

Our company issues Blackberries and I find it extremely valuable since I'm out of the office a lot.  I find they can be a bit addictive at first and there is certaintly some eiquitte involved in there use (don't break it out in a small meeting if your bored).    
I'm about to get the phone / PDA combo with color screen, be interesting to see how it works out.
Akro