We've gone VOIP at all 3 centres with a hosted CISCO VOIP system! As previously posted - quality very much depends on bandwidth! Same company that supplies our broadband does the VOIP - we're very happy with the quality of calls but not convinced that we save that much! We have 2Mbit SDSL links between our main site and the branches and 4Mbit out from our mainsite to the WWW. Considering we have up to 64 simultaneous workstations accessing the server over Terminal services with 48 phone handsets and 18 incoming lines, no complaints thus far about quality of call. And we've even started using the same BDSL for telehealth video conferences simultaneously!
To quote our IT people - whatever VOIP gateway you use needs to have a minimum qos for VOIP.