Sometimes it's hard to tell which mobile network offers the best internet browsing experience whilst you are on the move as mobile network operators try their best to keep secret information about their infrastructure, base stations locations etc.
Here are some resources giving some evidence of service levels and coverage of major network operators such as AT&T,Sprint and Verizon Wireless in the US and Orange, O2, T-mobile/3, Vodafone in the UK.
Ofcom coughs 3G coverage maps
direct PDF links: http://www.ofcom.org.uk/radiocomms/ifi/licensing/classes/broadband/cellular/3g/maps/3gmaps/coverage_maps.pdf
The 3G map Ofcom didn't want you to see
Not all 3G wireless networks are created equal