I think I have tried one of the cheaper ones (Malvern Audio/dappletoft) and could not notice a different. The original ones come from the Funk Firm - they also sound good. I may try an OL one, as a couple of people I know reckon they can hear an improvement.
I have the belt that came with my PT (said to be a PT supplied belt), an eBay cheapie, a Malvern audio and Origin Live versions. Not sure if I can honestly say any one sounds radically better than the other. There are differences in start up performance as some belts definitely slip more than others and yes I think they do sound ever so slightly different but I would suggest the differences are very subtle, not worth chasing and if you want your £30 to make you smile buy some more vinyl instead. Possibly heresy but the biggest single night and day improvement I made to my PT Export was changing from the AC motor to the Origin Live DC motor kit, suggesting the DC motor was the right philosophy from the outset.
Well I finally have my cartridge back after retipping (Esco did not receive my original confirmation email). I can say this new belt is much tighter than the old one, the PT was purchased 2nd hand, so I had no idea on how old the belt was. Well the new belt works fine, don't think I would be able to hear differences in belts, since the tt is sounding very good now.