Well the 8520 has incredibly slow internet regardless of which network you are on. All BB internet traffic goes through RIMs servers, and the 8520 doesn't have 3G support, so you'll be waiting a long time to do anything online.
I've been on Vodafone with my Blackberry for ages, and they are absolutely fine. The internet isn't greattt, but it's not awful. Some websites (BBC, wikipedia) take ages to load, but things like Facebook are fine.
If you are planning on using the internet a LOT, you should get a 3G enabled BB.