Recently moved from o2 for my iPhone contact to Three with the release of the 4S.
To say I'm impressed with the service is an understatement. Amazingly good value -- £99 upfront on the 'one' plan when I signed up, but £150 quidco back which was paid swiftly. For 2000 minutes, 5000 SMS and unlimited data it's a steal.
This compares very favourably with what o2 was wanting -- I was on a grandfathered unlimited plan for £40/month for 900 minutes and unlimited texts. o2 wanted to charge me £51 (!) for 900mins/unlimited texts and 1GB data.
Coverage is *vastly* better than o2 in Edinburgh and on the main train lines (to Glasgow; east coast and west coast). I wasn't expecting such an improvement with it being 2100MHz vs 900/1800 for o2, but they've obviously done something right.
I've also not had a single dropped call on three yet, something which was a massive issue at peak times on o2.
And the speeds on HSPA+ are very impressive indeed. I'm currently using my iphone as a hotspot (which is inclusive in the package as far as I can tell) as o2s home broadband service is horrendously congested. Getting over 950kB/sec vs 20kB/sec on o2.