I use PerfectDisk because it consolidates free space. It can also perform an offline scan at boot-time, allowing it to defrag some files that can't be moved when Windows is running.
After some research on the subject, I decided to avoid Diskeeper. If you google for comparisons between Diskeeper and PerfectDisk, you'll find some good information about how both tools work, and how they compare to the default Windows defragger.