Apple’s Camera Connection Kit offers two items that connect to the dock. One of them reads SD cards, but the other one allows you plug in a USB cable, letting you send a signal from an audio interface into the iPad.
Apple does not officially support using the Camera Connection Kit in this way, but you should get results as long as you have a USB audio interface that is class-compliant (ie works without its own drivers in OS X) and either requires less than 200mA of power, or has its own supply.
At its asking price, the Camera Connection Kit is the least expensive in the round up, although it does have to be paired with another device. Still, it could be the practical budget choice – if you don’t need pocket-sized portability and already have a compatible audio interface, why buy another? Just connect it with one of these, and you can stick your photos on your iPad too!
The Bottom Line
We like: Compact; lets you use an existing USB audio interface; inexpensive
We dislike: Won’t work with all audio interfaces