Yep, I have my Dell Streak on GiffGaff
your need to apply the GiffGaff settings to get the data connection working, detailed here: http://www.igaff.co.uk/wiki/index.php?title=Dell&redirect=no
It all works perfectly.
FYI: Whille I really like my Dell Streak, the 2.1 android build that O2 released (and later took down) is pretty darn rubbish, I'd say don't upgrade from 1.6 if you can until the 2.2 build comes out; latest rumour I heard was November for 2.2
Also, while the gorilla glass is strong, the screen underneath seems to have some problems with breaking, and the back cover over the battery is a known issue with coming lose and making the streak restart. so just take care of your Streak, get a case for it pronto!
Also the gorilla glass CAN be scratched. I have quite a long scratch across the front, thankfully its pretty hard to see unless you really look for it, but anywho you have been warned, get a screen protector.
Please don't be put off though, the streak is great, the 5" screen really kicks bottom, you just have to be a little more careful with it than your average phone and it will be fine.