Blackberry's, although better than they were, are still more of a communications device than a PDA. Some PDA phones are beginning to bridge the gap but are still short on the overall functionality of a PDA and are more expensive.
PDA's come as either Windows or Palm devices (there's some Symbian and other flavours out there too but they are more phones than PDA's). Palm is beginning to fall out of favour, though in my view it still has it in terms of display, speed and sheer wealth of applications you can get cheaply or for free.
If you want a device to create documents, play music, manage files, play videos, surf the web {limited but usable}, check mail, be an alarm clock, connect your phone as a modem, connect to WiFi, be a thesaurus, run navigation software etc.., etc... and you don't want a laptop then you need a PDA, it will do all of that plus more. Make your choice of a windows or palm device by finding the functionality and programs you need first and then seeing if it will do it. My bet is the Palm will still be the one you choose. Of course you have to be sure first that you can get on with such a small device, many can't so it's necessary to try one if you've never used one before.
Here's one website which does a lot of PDA reviews and gives some sumaries of what's available, there are many others.
http://www.mobiletechreview.com
If you want more of a phone than a PDA look at Orange's own devices, they get good reviews and are Windows based devices.