Depends what you are into, doesn't it? I spent 10 bucks on an app that turns the iPad into a control surface for Logic so I can have physically controlled faders on the mixer.
If you don't use logic, I doubt you would like it.
However,
Simplenote brilliant as it roundtrips with my mac and my iphone.
BeeJive is essential
AirSharing HD is great
AirVideo is probably the most impressive. Any videos in any format you have on a box and it will stream them to the ipad and do live conversion and it works REALLY well.