I've been with them for a couple of years on various phones and have had the iPhone 4 with them for a few months now.
I'm on the One Plan @ £35 a month for 2,000 any network minutes, 5,000 same network minutes, 5,000 text and unlimited internet which includes tethering - the iPhone only cost me £69. That's a real deal for what you get!
As far as reception goes I usually have reception in at least the same places if not more than my friends on other networks - there are going to be some areas that are black spots for each network so check the postcodes you spend most of your time at to see what the reception is like there (i.e at home, work etc). If you are really worried I'd suggest you get a Three PAYG sim and use this in your current phone for a few days to test out the reception.
The only downside is that in the UK only o2 offer Visual Voicemail but you can get around this by using the Hullomail app/service which is arguably better than carrier Visual Voicemail any way...
I wouldn't be on any other network to be honest.
If you have any other questions let me know.