Monday Night Gaming Group has been using Fantasy Grounds 2 and Skype for several years. FG is very pretty, allows for sharing of multiple maps, and the rule support is excellent (a real bonus with GURPS). However, we have lost a few nights and have had many delays because of FG issues. We have also had Skype issues.
With the Montporte Dungeon Campaign, I have been using Roll20 and we are getting more acclimated to it. It works particularly well in conjunction with Google Hangout. It is not as pretty as FG and it doesn't have all the functionality of FG. However, for our d20 game (Blood & Treasure), it is simple to use, reliable, and facilitates play.