I have wanted a blackberry for some time but being on O2 PAYG I opted for an HTC phone. This was a mistake it had limited functionality and wouldn't let me download Apps. So when I heard about the Blackberry 8520 I jumped at the chance especially for £139. I have never been a fan of the finger swip interface and wanted to return to the old fashioned button menu and the blackberry works great. The only problem I encountered was the O2 PAYG package I had did not include the Blackberry BoltOn which is £5 a month. Once this was setup I could configure my Blackbery to recreive and send email through my hotmail and gmail accounts.
This phone works like a dream it fast intuitive and works very well with email and SMS, the keyboard is small but I don't have any problems using it (not like the HTC, with my fat fingers). The keys are rounded so differentiating between keys is not a problem.
Pros
Email
SMS
Scroll Button
MicroSD memory expansion
Configure Wifi to send\recieve email
Cons
Blackberry Maps (not has good has google maps)
Camera is low res and the digital Zoom is pointless. When you zoom in the picture size is reduced and pixelated.
If you use email, facebook and twitter then this is definitely the phone you will need.