This is just a personal experience as I know others are getting on fine with giffgaff.
My main issue is that I keep losing my connection to the Google servers ie my signal icons keep turning white. Have checked all apn settings and not that. Tried a different ROM, kernal radio etc and not that. Unrooted back to stock and not that.
Have posted on the forum and got about 100 replies telling me to do what I already stated I'd tried in my OP. Sent numerous messages to giffgaff and not received one reply.
Personally think it must be down to the area I live but it is strange because I'm getting a good strong constant H signal. Also ordered a new SIM and doing same on that one.
To top off my giffgaff experience it took me 5 days to top up a goodybag because the site was having issues. From reading the forum seems like this is a common issue.
As I said this maybe just me having a bad experience but not having a good time