I moved from Virgin to 3 a month ago - the 12 month £15 SIM only contract. So far I've had no problems at all and my download speeds are generally faster. My sister moved from O2 to 3 at the beginning of the year and has had the same experience. Her d/l speeds at home (Uxbridge, Middx) are usually around 24mbps and mine at home (only 2 bars of signal, Whipsnade, Beds) are around 10-14 mbps. About one call in 60 or more I've had a slight echo my end (calling from inside a car in a multi-storey car park) but otherwise all is good.
I would do as someone else suggested, sign up for the 1 month £18 plan and try out the coverage in your area, alternatively try a PAYG SIM for a month (I think it cost £15 for a top up that included unlimited data for a month (no tethering), but I can't remember how many mins and texts you get.