Three all day long. I had read the bad reviews also but made the decision to switch to three from o2 8 months ago, and its the best decision I have made. Coverage is better and the data speeds are fantastic.
I've just got the 5 on Threes 'one tarriff' and its £36 a month for unlimited date including tethering and I get 2000 anytime mins, 5000 three mins and 5000 texts, more than enough considering most friends have iphones so I use iMessage or whatsapp. The handset was £79 too which is by far the cheapest offer. Three have also been given permission to use the LTE band from ofcom so during the next 12 months their 4g will be up and running, in the meantime though their 3g is faster than EE can offer on 4g.
Customer service has been excellent so far, even when I went to spain last week I forgot to enable euro roaming, three sorted all that out for me and put me on unlimited data use for only £5 a day.S
I cannot fault them at all...
http://www.three.co.uk/Store/Phones/iPhone?intid=3mainhprbn579