Having used o2, Orange & Vodafone (& had differing issues with all of them), I am now on a monthly rolling SIM-only plan with 3. I'm a heavy data user on my iPhone (constantly on FB/Twitter & Skype video call my son abroad a few times a week), so have the unlimited data plan - think it costs me about £20-25 a month. It also has more than enough texts/minutes per month as well.
Was easy to set-up & activate & they've been pretty helpful when I've needed to call. 3's network is also increasing quite rapidly, so coverage is fairly decent for me in most places in/around London where I now live. The only signal issues I've had is when travelling on the train for work between London & the Midlands & Manchester. The 3 app also gives you your usage stats, but is a pain if you want access detailed data or contact them, as it then diverts you to the website via Safari.
All in all, I'd defo recommend them over the other 'big' networks.