I feel a little bad only just getting to this months RPG Blog Carnival hosted by Gaming Ballistic, but it's a good topic, and one definitely worth addressing - Virtual Table Tops.
While I've done a fair bit of gaming over G+, I've only recently begun to use a VTT. Everything prior was just the table top of the imagination, as things were described, rather than shown. It's just a step removed from being at the table, really. The hardest part can be figuring out everyone's voice, and showing things on the camera if you have a map or a picture. The virtual table top kind of addresses that last issue...
So what about that? I haven't run a game with a VTT, but as a player I've used Fantasy Grounds 2. Getting to see the battle map, with the monster and player tokens, getting to see the images that the DM has picked out to illustrate the various NPCs, the world map, all of it is just the biggest advantage to using a VTT over just using skype or G+.
On the other hand, if everyone isn't really comfortable with the program it can slow things down. I've definitely noticed that, being a novice with the program. The other thing that I've noticed is that with the chat feature, when other PCs are whispering with the DM, it's easy to get distracted, and since typing takes longer than talking, what might take a moment in person, takes much longer. So when the DM and a player are whispering everyone else is just sitting on their own computers, waiting.
Overall, I think with VTT's can bring a lot to playing online, and I'd recommend that DM's running online games check it out.