Many people have issues with the internal flash based voice chat. Almost every group I have been in and that has been a lot someone can't be heard or can't hear others. Google Hangouts is another popular option besides Skype, especially if you plan on recording or streaming your games. In fact I have teamspeak, mumble, raidcall, skype, and a google+ account all installed on my laptop just so I have my bases covered for whatever the group wants to use. To be fair to roll20 I do like the WebRTC but for some reason I think because it is not the default voice service people in most groups still rather use Skype or Google hangouts.