Firstly, some people say that Defraggers are not worth paying for but I use Diskeeper 2007 and IMHO yes, it is worth the [relatively little] money asked for.
I have tried freebie defraggers but found these cumbersome then a friend gave me a trial version of this and found it much more user-friendly and less complicated. It may just be me but the speed up after the initial defrag WAS noticable - although I cannot honestly say the same about later defrags.
It defrags in the background and allows you to continue to use the PC while it is active. I also have the scheduler turned on so there is nothing more for me to do and this also checks the condition of my hard drive and is set to warn me if there is a problem found.
Yes, I think that this is a lot better than the Windows-included version.