Please note that this is not a scientific study in any way. It's simply my personal impressions of some of the defrag programs mentioned in this thread.
Defraggler was very fast. It finished working on my 160GB hard drive in less than an hour. Unfortunately, according to other programs (including the built in XP defragmenter) the drive started at about 25% fragmentation before running Defraggler, and ended up with nearly 60% file fragmentation after the program was finished.
O&O has an interesting feature that no others have that I have seen. It includes an option to run CHKDSK on the drive before defragmenting. I like this idea, except that it errors out and crashes the program every time I try to run it (this happened on two separate computers). Defragmentation takes about the same amount of time as the built in Windows utility but includes the ability to select how the files are reorganized. This seems to be a good program, but it's not free and that's a major con for me.
I like the concept of UltimateDefrag. I particularly like the way it (claims) to manage the placement of files on the drive based on how they are accessed. And the round cluster display is an interesting variation from the standard rectangle of blocks that all other programs use. However, it is EXTREMELY slow. I was curious to see how my system would perform after the hard drive was completely reorganized using the supposedly unique placement algorithms so I have let UltimateDefrag run overnight for the past two days on the Auto (by file access) setting and it still is only about 50% done with my 160GB hard drive. I fully expected it to take a long time to finish since this drive was badly fragmented after Defraggler mangled it, but to only be halfway done after about 20 hours is pretty excessive..