I don't know about MyDefrag, but I once tested UltimateDefrag and it corrupted my harddrive within one day; "luckily" I had a full backup (not really due to luck).
Regardless of what any software may provide in terms of speed and features, first and foremost it must NOT destroy your data!
I seriously doubt, however, that all defraggers except one will corrupt your harddrive over time - unless you have overclocking or memory problems that would corrupt your harddrive anyway.
My current favourite defragger is Smart Defrag, but I also use others such as e.g. Defraggler for defragging single files and PageDefrag for defragging system files (during boot).