WABES2K  Also, someone commented earlier that after defragmenting their hard drive with iDefrag, Time Machine backed up all their data as if it was all new. I have not had this issue with either version 1.x or 2.0 beta.  
(Version 2.0.0b2)

WABES2K  Version 2 looks really nice! In the old version, I always had to burn a bootable disc to run a full defrag on my startup disk. I did this each time because there would be an update to the app by the time I defragmented my hard drive again. In the new version, that is still an option, but is no longer necessary. When you run a full defrag on your startup disk, iDefrag simply restarts your Mac into a special mode so that it is the only app controlling the hard drive. I'm looking forward to getting version 2 when it comes out!

I ran into an error telling me I had insufficient movable free space, when in fact I had 86GB of free space. Turns out I had an error on my drive (which was not the fault of iDefrag). I just had to run repair disk (available from the "check for errors" menu option in iDefrag) to fix it before I could finish defragmenting my drive. I initially submitted this as a bug report, and I can say that their support staff were very prompt with their replies and helpful.

There is a lot of differing opinion about whether a Mac HD needs to be defragmented, but I definitely like to do this once every several months. Whenever my Mac gets pretty sluggish, I know its time to defragment my HD. My apps really are much snappier afterward. As much time and frustration as its saved me, it is definitely worth the money.  
(Version 2.0.0b2)