Disclaimer - the folks at Diskeeper gave me a free copy of Diskeeper 2008 Pro Premier. But, I've purchased previous versions of their product.
I've been using Diskeeper for a couple years now, and I've got to say, I love this product. Back before Vista was released, I had purchased and installed it on my Win XP Pro machine, and compared to the Windows Defrag utility, well, there is no comparison. Where Windows Defrag takes forever to run, Diskeeper was finished in no time. When I put Vista on my laptop, it was going back to the defrag dark ages until I got the new Vista ready version of Diskeeper. Since then, they came out with Diskeeper 2008, which just keeps improving on a great tool.
Diskeeper for Home/Home Office comes in 4 versions, Home, Pro, Pro Premier, and Home Server. The Home Server edition is something that is going to be sorely needed, a version of a desktop utility that needs to run on a server, but not at the price of a server version. The Home version is $29.95, and the Home Server version is $69.95, which is a very reasonable price. When I was beta testing Home Server, one of the issues I had was the price of things like Antivirus and defrag utilities, because it is basically Windows Server, which most desktop OS utilities will not run, and the price of server versions are a lot more than what a hobbyist will want to spend.
I have a HP 8510p notebook with Vista Ultimate installed, and Diskeeper is one of the first programs I installed. As a developer, I tend to install and uninstall lots of programs. Combine that with Source Control and versioning, and my harddrive tends to fragment pretty quickly, so a tool like Diskeeper keeps my drives from getting fragmented and performing up to their potential. The only issue I have with Diskeeper (and it isn't so much of an issue with Diskeeper) is that it doesn't integrate with my Security/Anti-virus Utility of choice, Windows Live OneCare. OneCare likes to schedule "Tune Ups", which basically consist of running things like virus scans and defrag. The problem is that OneCare doesn't have a way to let it know that I don't use the Windows Defrag tool, and use Diskeeper instead (OneCare has the same problem with Backup Utilities). So, it during the tune up, it tries to run the Windows Defrag Utility, which is slow, and doesn't seem to like the way Diskeeper defrags. I also am not a big fan of the amount of time OneCare takes to scan my disk for viruses, but that is a whole other post (but OneCare is still my favorite Vista Security/Antivirus Utility). Other than that slight issue, I love Diskeeper. If you are a developer, or just abuse your harddrive with lots of new/updated/deleted files, Diskeeper is a must.