I've had 80% success with this. (1 out of 5 was totally screwed).
You can open up the drive and free the head, typically it's just stuck and needs a nudge.
However after opening one up, it'll be on it's way out. I've only done it as a free last ditch attempt. If you really want your data, then send it off to a data recovery firm.
Just buy another and restore from your backup.