Heres my story:
I had a spare LG310 (actually 2 of them!). Got 1 unlocked and its now working with o2, BUT and its a very big BUT, o2 does not support this LG310 and if I request WAP and GPRS settings they never arrive. I've tried manually editing the GPRS settings but the phone insists on using 3 default settings only. Also I can't get rid of any of the 3 stuff on the phone, so it looks like I've only got calling and texting capability on o2. Ah well, £20 unlock fee was still cheaper than buying a new phone.
Now, the other LG310, I got the 3 SIM card this morning (with £1.02 credit), I've already set up Windows Live messenger and Skype with no problem at all, and most important of all, I didn't need to top up. So far so good. Now if only I could get 3g speeds where I live that would be great.