Hal,
I've helped implement NAnt at two different companies now, along with NUnit and CruiseControl at one of those companies. I find each of them to be easy to use and very well supported by a robust user community which is key when choosing open source solutions. One of the things that I really like about NAnt is that it is easily extensible. I've had to create several custom tasks in my current job. I was able to get those custom tasks created quickly and easily.
I'll second Bevan's advice to you when it comes to implementing things little by little and documenting things carefully as you go. I'd further that advice, in terms of NAnt by suggesting that you come up with a template that you can use when creating build scripts for new projects. I've found great efficiencies in keeping all of my build scripts similar to one another. I've also found good benefits in creating parent/child hierarchies in creating build scripts. The parent script basically controls the overall build and the child scripts simply build at the assembly level. Doing so allows you to easily integrate those child build scripts into different parent scripts. Finally, I'd point out that one of the best ways to learn a lot about NAnt is to look at the build scripts for NAnt itself (and other sources like NUnit that are build via NAnt scripts).
One word of warning is that as easy as you try to make everything, you may find that people are simply not interested in the build scripts in the same way that you are. In both instances where I've implemented NAnt, I found that the other developers simply didn't want to be bothered with the scripts which meant that I ended up supporting them. Then again, that may just be a function of the kinds of environments where I've tried to implement NAnt (one dysfunctional and immature and the other overly complex).
Hope this helps.
