I must confess that I've never used a virtual tabletop. I understand that they're very popular, though (Roll20 says it has close to a million users, and it won an ENnie last year). I just got sent a link to a press kit for Roll20's new mobile apps for iOS and Android. The short version is that Roll20 is now available on Android and iPad. The long version is below, along with a nifty video of the iPad app in use.
[video=youtube;rxbOlPKF_e0]https://www.youtube.com/watch?v=rxbOlPKF_e0[/video]
[video=youtube;rxbOlPKF_e0]https://www.youtube.com/watch?v=rxbOlPKF_e0[/video]