I certainly have one and have tested extensively with it:
- Disk image mount and unmount just fine.
- Speed is quite slow, about 1-1.5MB/s wired. (Compared to an Infrant ReadyNAS NV+, which runs around 3-6MB/s wired.)
- Seems to run fine scheduled on wake.
- You can also share files, yes... but you can't use SD! to back them up.
Best practice: do your initial backup to an image while actually connected to the real drive. Then point SD! to the same image (it's grey, but you can click it) on the Airport Disk when you attach there.
Be aware that this is a 1st generation product... and, if you can afford it, there are far, far better NAS solutions out there (the aforementioned ReadyNAS NV+ is, by far, my favorite product in this space).