Windows defragmenter sucks. Look at this image.
This is the drive map generated from Diskeeper after defragmenting with Vista's inbuilt defragmenter, which is supposed to be better than XP's. I don't know if it is really true (that Vista's defragmenter is better than XP's) though.
There are added features. For example, i-FAAST in Diskeeper will monitor the frequently accessed files and rearrange them to get the best performance. It also has a I/O monitor so whenever we need to read/write to the hard drive, defragmenter automatically pauses so that you can work normally even when the hard drive is being defragmented. Because of this, I no longer hesitate to defragment my 278GB data volume.