I have used all these providers in their trial mode. Has anyone used them for real (ie afer paying the credit) and have there been any differences in quality? I mostly make UK landline calls so the call rates aren't going to affect my decision.....

Personally I'm using Voipstunt. Despite the fact that they are all branches off of the same fineara tree, voipstunt appear to have a greater selection of free countries. Look here for a comparison.
http://www.myvoipprovider.com/Free_VoIP/

Since paying my money, I've not noticed any improvement in quality. But then it has been good since the beginning. Occassionally you get an echo, or the effect of talking through a bucket of water, but just hang up and redial and it usually gets sorted.

Perhaps when I get a decent connection, I'm only on Telewest 10mb, then it might improve. Roll on the year when TW & NTL bring their 100mb service