O2's 2G network is
very good, as is the customer service. 3G however is a mess.
* Only 85% population coverage (O2's own figures)
* Mostly HSDPA, not HSPA+
* When 3G is not available you don't even EDGE in a lot of cases, just GPRS which with the average web page these days, could take 3 Minutes+ to load.
* Many hops in O2's network before finally sending your request out on the internet, and many of those hops very latent - evident in a traceroute, not the same on other networks
* Doesn't even perform that well in coverage areas, intermittent
* Very scaled down grainy images on websites that make your £500 smartphone with retina display look like a cheap knock-off from China, a.k.a screen quality for web browsing that matches the '
airphone 5'
I think that just about sums it up ! I know somebody will come along as say it happens to work where they live / work, but that's my experience of testing and comparing O2 against other major networks in various places.