Same issue on my Telus HTC One X+ here in Canada. Tried the phone on three Bluetooth systems (two ear pieces and one car speaker phone), all sounds terrible on my end with lots of background noise. The individual on the other end says that I come through loud and clear though.
I think that it's safe to say at this point that this is a software issue with Sense 4+ on the HTC One X+ and that all units are affected.
Personally I am going to keep my One X+ since I don't actually use Bluetooth very often, but I can see how this would be a deal breaker for lots of people. I will call HTC on Monday and also report my findings in order to help escalate this issue.