as far as i know, blackberry service isn't available on pay as you talk on any of the networks. if you're not going to use email thats no big deal BUT, with most networks, if you don't have the balckberry service, you cant use mms or GPRS for internet access either, which takes away the point of having one.
Blackberry's are great for messaging by text, email, IM (blackberry messenger is great if you've got mates with blackberry's as you can chat for free!) but if you're mainly going to be using it just as a phone, there are better alternatives.
The media player is pretty good, but will take a little getting used to. The camera is pretty good too considering its only a 2mp.
If you are going to invest on one for use on prepay, check with your network first, as i'm pretty sure that o2 will insist you're on a business contract to use blackberry.
If you want push email on your phone with out paying extra costs, look at a Nokia phone running Symbian 60.
Push Email on Nokia Phones
These will work on PAYG no problem, but you'll have to pay for the data transfers.