I'd also be interested in any feedback on this.
Having had enough of being screwed around by BT, I want to cut landlines out of the equation altogether. 3's option seems ideal on paper.
Apparently you do get three days to try the thing out and if you're not happy, they'll cancel the contract, but three days isn't a huge amount of time.
My other concern is that 3 are not renowned for having great cover. The joke used to go, "Hello! I'm on a 3 phone and touching the mast! Can you hear me?"
Having said that, they do postcode checks to ascertain what the cover is like where you live.
I've been putting a bit of research into this and the next most appealing package for me is T-mobile who offer 3gb and give you a more generous 14 days to try the thing out before the contract becomes set.
They are naturally a bit more expensive though.