<p>I currently use nant, ccnet (cruise control), svn, mbunit. I use msbuild to do my sln build just because it was simpler to shell out. </p>

<p>Are there any merits to switching my whole build script to MSBuild? I need to be able to run tests, watir style tests, xcopy deploy. Is this easier?</p>

<p><strong>Update: Any compelling features that would cause me to shift from nant to msbuild?</strong></p>

I feel that MSBuild and Nant are fairly comparable. If you are using one of these, I generally wouldn't switch between them unless there was a compelling feature that was missing in the product you had selected.
I personally use MSBuild for any new project, but your mileage may vary.
Hope that helps!
Edit:
@ChanChan  - @Jon mentions that Nant doesn't build .NET 3.5 applications. This may be enough of a reason to either change, or at least use them in parallel. As I've moved more towards MSBuild, I am probably not the most informed person to highlight any other showstoppers with either technology.
Edit:
It appears Nant now builds .NET 3.5 Applications.