It's basically a matter of personal preference. Defrag is Defrag, its not rocket science. One of the major bonuses of 3rd party Defrag is speed. Always, in the past windows versions anyways, stock windows 98SE (last time I used windows defrag) would take over an hour on my 240Gb hdd. Norton would do it in 20 minutes.
Not sure about Windows 7/8, but I Defrag my 1Tb in less than 3 minutes with Norton now, which has 5 partitions.
If my Norton wasn't free, I'd probably be tempted to use piriform, or even windows, but Norton for me is easy and painless and comparatively very fast compared to the Windows predecessors.