I have used both windows 2000 professional and windows NT 4.0 workstation. SPAMThe main advantage of them is they are built off of the NT kernal so your system will be very stable. SPAMWindows 2000/NT supports a lot of server and high end user options. SPAMThe only disadvantage is that it is a little harder to set up and use.
It's crazy I'm thinking, just knowing that the world is round.
-http://www.techwarepc.com/ - The Technology Experts
Windows 2000 is time consuming to convert if you use a wide variety of software applications that are non Microsoft applications. They do have a program that will identify applications that are not compatible which SPAMsaves A LOT of time. An issue that arose for me was if an application is not compatible, you are stuck with finding(and buying) something else or trying to get the manufacturer of the application to make necessary changes.
I did find Microsoft support team willing and able to help through the transition as well as helping with working with vendors to get changes made.
Another dis-advantage is that it takes longer to boot a pc with 2000 - of course this could be considered an advantage also because it is a perfect opportunity to go get my morning coffee!
Heck, I find windows 2000 to be really cool. Like the others said, its way more stable. It's pretty much the same as Windows 9x for basic everyday use. It is a little harder to install and setup but not a biggy if you know computers. As for software, I find it runs quite a lot - it doesn't have to be Microsoft. However, 16-bit applications seem to run slower on win 2000 than on Win 9x machines. You just gotta try it.