I used to use HTB a long time ago in the Linux box I had as a router. The reason I switched to HFSC was that it performs better for VoIP applications, HTB reorders packets very frequently while HFSC doesn't! It's a difference you'll never see while just downloading data but makes a hell of a difference in VoIP apps that are very sensitive in the order that the packets arrive, out of order packets create jitter and that causes delays since the application has to buffer more.
The problem with HFSC though is that it is much more complicated to configure so you won't find it often in generic QoS solutions.
Hm, never had any issue with VoIP here, and I even spent about 2 hours on the phone two weeks ago doing a server migration
.