I consider Revo Uninstaller to be a must for uninstalling programs from my computer. Using Windows' built in uninstaller utility will often times lead to left over pieces from the program being left behind to clutter up the system. Revo Uninstaller finds these pieces and removes them as well, preventing this cluttering.
When you select to remove a program through Revo, it will first ask what Uninstall mode you wish to use. Using Built-In Mode does the same thing as the Windows uninstaller, which would make Revo unneeded. Safe Mode will remove any left over files while Moderate and Advanced Modes will remove left over files and registry entries (Advanced runs a deep scan of the registry).
Revo first uninstalls the program using the built in uninstaller, it then runs a scan for leftover files. Most of the time I haven't had any issues with the file scanner. On occasion, though, Revo will sometimes misidentify registry values and files if your computer has something similar to what you're uninstalling. I only use advanced mode, so this may not occur in the safe and moderate modes, but I would suggest manually checking everything before deleting it anyways.
Revo also contains a Hunter mode which allows you to drag and drop the program that you want to uninstall onto Revo. It will then find its uninstaller and proceed as described above. Unfortunately, I've had some issues with Hunter mode. Lately, no matter what I drag and drop, it always tries to uninstall OpenOffice. If you try using this mode, you should probably check that it's actually uninstalling the correct program.
Aside from uninstalling programs, Revo Uninstaller also has ability to remove unneeded files from your computer among other things. As I use CCleaner and nCleaner to perform these tasks, I have no experience using Revo's tools and will exclude them from this review.