My Take
GarageBand is a creative app, which can be applied to a wide range of instructional material, so if used effectively it is a great tool for teaching. Yet like many other creation apps, it take some understanding of the tool to get the best results. Your students will need help with getting the project put together, and if you put in the time to learn the basics, their projects will soon blow your mind!
The only other concern is that it does take up a lot of memory. The samples, sound effects, instruments and effects on the program will take up a lot on their own, let only each project that students save. It's best to have a central computer (Apple of course) to back up projects, thus freeing memory and speeding up processing time.
How I Use It
My students always enjoyed singing and music, once I showed them this awesome app they went nuts! If you get this you need to unlock all the sounds, instruments and samples- endless possibilities! Besides making what you might call songs, we used it for creating narratives for digital stories, podcasts, audio books (with sound effects, adds an awesome element to a student reading aloud). It's also a great app to just play on, I've found myself engaged in it for hours only to produce, in my mind, an awesome 15 second musical journey, though my wife always thinks differently!
A MUST HAVE for all classroom iPads!