If you're going for an internal, just be sure to get one with correct height. As far as I know, the 15" MBP has a 9.5mm height optical drive (which is quite unusal sometimes called "SuperSlim", I believe), while the 17" comes with a standard 12.7mm height notebook drive that should be widely available.
Well... I'm pretty sure on this, having it read or figured out from some sources on the net. But let me say that I haven't checked or installed these drives myself yet (One is being shipped to me at the moment). So you might want to double-check this, depending on your MBP model. Especially if you 17" model which has fewer reports on drive replacements.
As for the actual install, just make sure you follow the instructions (ifixit.com for instance) meticulously and take your time, so that you won't break anything. Then it's really not so difficult.