I have a Roland Duo Capture EX for audio interfacing w/ iPad.
Back when I was planning my iPad purchase, I considered the Alesis IO Dock. Then the iPad 4th gen came out and it used a different dock connector than the IO Dock! That was when I decided to avoid anything that encloses the whole iPad, because once you spend that $150 or whatever for an IO Dock, you can only use it with the iPad that it fits - if for some reason you upgrade to a new iPad model, it might not fit at all, or the connector could be in the wrong place.