I defragmented and repartitioned two drives (one having Leopard and Tiger partitions, one having Leopard and Windows FAT partitions), first with iDefrag (full) followd by iPartition. This worked perfectly, albeit slowly. I lost no data and there was no problem.

I did, however, as recommended in the FAQ, do this from a bootable DVD I prepared first following the instructions rather than from the hard drive.

And I had a complete backup first - as one should always before repartitioning, and as is also recommended by the data manufacturer. In case of a problem, data loss is then impossible - only time loss.

But there was no problem.