My only question would be why? I mean, if you just want an extra drive, go ahead, but what improvement would it make? I could be totally wrong here but I think Macs, esp. the iMac, comes with everything you'll need hardware wise.
Same reason you'd get two superdrives in a Mac Pro. You can burn two at once, you can be watching a DVD while ripping a DVD or burning a disk. I find it very helpful.
According to its page on the Apple Store, it requires a MBA, no other computers listed as supported. Not sure if this is true or how it would be enforced.
I got my macbook on clearance at my school book store. Unfortunately, it has a combo drive. I would love to use this as an external drive, or even crack it open and install the drive internally if it's the same size.