I haven't had an O2 contact for over 9 years, I think O2's infrastructure is creaking under the use of Iphone app usage and increased customer demand for mobile broadband. I think challenges lie ahead for all mobile service providers to ensure consistent speed and availablity of 3G and HSDPA connections. It will require increased investment in masts, repeaters and bandwidth.
Hopefully someone will be along with experience of both t-mobile and O2 services.