I'm very satisfied with iDefrag: it feels solid and reliable. But there's a couple of enhancements that would really make it a perfect tool.
1) Some automation would be nice. You have to stay there until the dialog pops up, asking you for the password. Then it asks again for the password at a later point in the process. All in all it's a bit too manual: I wish I could start a complete optimize and then leave the Mac alone for a few hours. Also, I wish I could fire up optimizing of several volumes at once (although nobody offers such a feature on OS X).
2) I wish there was a graph showing the fragmentation state for each volume, and I wish I could keep that graph open while I go and check other volumes.
3) You can only deal with one volume at a time, and then, for each volume, you must enter the password, then wait for iDefrag to read the contents. If you decide to leave it there and go to another volume, same procedure. Now if you go back to the first volume, same procedure again (iDefrag forgets that it already read the contents 3 minutes ago). Can't you guys make it faster and easier for the casual user?
Otherwise I'd give iDefrag a fifth star.