I do it regularly, and it works quite well. Just remember, it's a "board mix" and it won't sound quite like what you hear in the room because you're hearing everything in the room with your ears, while the mixer is only "hearing" what's mic'ed or plugged in to an input, and the mix may not be great because of that. For practice reference, though it's usually good enough. The only thing you'll have to deal with is starting and stopping the recording from the docked iPad.
Once you're done, there's the trick of getting the files off the iPad, but that's fairly well outlined out in the interwebs -- I found that pretty easily with a Google search.