I have been a Slingbox owner for quite some time, and I've honestly used it for an extended period of time on a relatively few number of occasions. Truthfully, the application has been more eye candy and a cool tech gadget for me, while other users, especially those who have to travel for a large part of their work swear by the application. With the release of the iPad application, I can see using my Slingbox much more frequently, and for reasons that are more than just for show. I do wish that the application supported multitasking, but I can imagine how this would really bog down the iPad if you had video streaming floating in the background. Apple TV (sound only) does seem to be supported, but I was unable to test this due to our basement being in disarray.
At our house, we have one television, and it's located in our recreation room. We choose not to have them elsewhere, including in our bedroom. However, the bedroom is quite the trek from the rec room, and there are times when I just want to catch the last five minutes of a ball game. With the Slingplayer Mobile for iPad, I am able to use a large enough screen (I just couldn't watch more than 5 minutes on my phone), with a reliable connection, and have a high quality picture to discern some of the more intricate details on the screen. I'm sure there are others in a similar situation where having access to your television through the iPad would be immensely valuable.
SlingPlayer Mobile for the iPad is available in the App Store for $29.99.
NOTE: The software application only works with Slingbox SOLO or PRO-HD, and the hardware needs to be purchased separately.