3 are the company i work for at the mo and if you want an honest opinion ill give it too you.
the network were utter cr4p when they first started up and at the time i was working for a generic provider and didnt expect them to still be about, however they have now been going for nearly 8 years and offer just as good a service as other networks. signal is roughly 96% across the UK but obviously there are big black spots, like other networks.
Our signal is better than some but obviously much worse than others and will be what brings you down with the iphone as its a phone you want working online and internet wise 24/7 which i cant really guarentee with our network.
Call centers are dubai and mumbai, very knowledgeable if you get a more 'human' person but trust me we have to deal with the same idiots as customers from tiume to time and they do grate on my nerves if they are s**t and sadly there are a few, but once again likewise with other networks.
Our deals for the iphone4 kick arse and we do offer a completly unlimited internet bundle with a couple of the tarrifs.
Shop serivce is pretty good for most stores ive had dealings with however as its an Apple product they will be your first point of call if theres a phone issue, but even though it may be a pain they do know their products inside out, much more than anyone in a differnet shop.
With 3 you dont really get a 14 day trial period unless you buy it online/over the phone which can be a git if you get it home, use it a few times then realise its not doing what you want signal or phone wise, however if its a signal issue and we have told you its fine were you are your well within your right to terminate
All in all, we have our faults but we have great plus sides. i would say after 9 years in the mobile business that each netowkr has its good and bad and none reallllllllly outshine the others.
If you want to know anything else or advice etc just pm me and ill be happy to help