Overpriced for what it does..
The author should bundle both iDefrag & iPartition for under $40.
I can't justify paying over $50 for this.
With that said, it works as advertised. I went from having 4 partitions with various OSes down to a HFS+ & NTFS partition without losing any data.
You must absolutely compact files with iDefrag prior to growing or shrinking a partition or you can kiss your partitions good bye. Mac OS X handles fragmentation fine with very little performance hit. That doesn't mean your volumes aren't fragmented.
Someone several posts below claimed he was able to change the partition map from Apple to GUID on a drive without losing data. Correct me if I'm wrong, but you will lose all data if you intend to use the drive for booting, as a bootable GUID map needs 200 MB reserved for EFI at the beginning of the disc. It may work for an external non bootable drive. I still wouldn't recommend it.

I wouldn't touch diskutil to do this with a ten ft pole. This app on the other hand is easy to use in comparison.