MbUnit or not
I was pointed at this post by Ayende Rahein on his move to NUnit from MbUnit. It's always sad to see folks leave MbUnit behind when the future is so bright but folks have there reasons. I will try here to look over some of Ayende's reasons and whats in the pipe line.
there is a wealth of tool support for NUnit, and very little for MbUnit. Mainly, I was moved by easy CC.Net integration (yes, I know that MbUnit has that as well, but not built in and not as easily)
As far as I know MbUnit supports every single tool that NUnit also supports (if thats incorrect then let me know what we work we have to do), CCNet integration is something I personally did so I can answer that its very easy to do. The MbUnit installer includes a stock replacement for the unittests.xsl and I have added a replacement for tests.xsl that is available from the CCNet wiki. Obtain these files and replace the stock files and your done. Easy as that.
There is also this post that shows your how this is done with a custom plugin.
Beyond that, MbUnit ceased to give added value over NUnit some time ago, so in the end, I had to ask myself more Why? than Why Not?
I don't believe that is the case, NUnit has a great plugin in model that allows you to extend it but MbUnit has all that work done for you and also can be extended with more. The only thing I can suggest to anyone that shares Ayende's feelings is to join the MbUnit community, join the lists, start talking and sharing your feelings and see what we as a community can do to help.