I am on GiffGaff.
The network can drop off, has a couple of times in the time I've been with them but its not been for that long to be fair. Its not a business phone so if i get knocked of for a couple of hours its too bad a problem for me. For what you get its awesome in my view, girlfriends contract with )2 ran out and she asked what she could get and they said, just what's online. So she went to GiffGaff and has no problems on her Iphone.
Although unlimited, no tethering on the mobile sim cards, not sure if thats a problem?