I use my iPad for some illustration and graphics work for 2D games, and for the most part, it is adequate. For illustration, I tend to use Autodesk Sketchbook Pro for illustration, sketching, painting tasks, because it has some great features, is easy to use and works well. If I need something that will let me get ideas down really quickly without getting in the way, Adobe Ideas is the way to go.
For animation, you are pretty limited. As far as 2D game animation goes, I have had some success using SpriteSomething for animating sprites and basic animations, and with the most recent update supporting zooming, I want to try it out on a more complex animation than the usual walk/jump/attack/die animations that I have done in the past. The only other animation program I like is Animation Creator HD, which is pretty great. Some of the brushes aren't the best and I have run into some lag when using it (this is on my original iPad), but on the whole it is a fantastic application. That being said, I would never really use it for production work. I would prefer to use Flash or AfterEffects for production so that I have more options, effects, an asset library, etc. But the convenience of being able to animate and test out new ideas and do great pencil tests with full color on the go is absolutely mind-blowing, and for that I thank Red Software for making AC HD.
While the iPad is great, the one thing I can't get over is the lack of pressure sensitivity for illustation. I would love to see someone come up with a stylus that senses pressure in the tip and offers and API for developers to take advantage of that data. And, there aren't really any satisfactory vector applications that I have used yet for iPad, so those cute icons and mascot illustrations have to be saved for Illustrator.
I haven't tried any android tablets, so I can't attest to that platform's offerings in the art field. One of my buddies does use Photoshop on a tablet laptop with a pressure sensitive screen and has been generally happy with it, but I have yet to play around with it.
Hope this helps.
