Three never owned any 2G equipment - they used to use Orange's network for 2G when their handsets couldn't see a 3G network (but this was invisible to users) but then they got all high and mighty over the price and did a network share with T-Mob and then that joint network will merge with Oranges in 2011 hopefully.
Three have a reputation sorta like virgin media - when it's working its working well, fast and without issue. If you have to call their callcentre though you'll likely need your firstborn sacrificed in order for them to do anything to fix a billing issue.
From a technical perspective their network is well designed and fast though, not sure if that really helps