Depends on what your doing. DC project won't see an improvement, but startup times and file access times should improve. If you install Diskeeper on a new built computer, its unlikely you will see a lot of an improvement, but it will prevent your performance from degrading due to fragmentation.

Also, remember the days of FAT file systems and crosslinked files? Much of that was due to file fragmentation. NTFS is far superior to FAT, but why find out where disaster will strike.