Vote for Voda
About 3 weeks ago I opted for the Vodafone 30day contract at £15pm and so far its been stonkingly good across the south east (mainly Surrey and Sussex). I've also used it on both a Win 7 and MacOSX and I agree with the easy interface, I even taught my otherwise non-tech fiance to use it on both.
I did find that a software update was available via the Vodafone website and that only improved things even more!
Well worth the investment if you don't want to commit to a lengthy contract.