VS2005 and /MP

After reading about multi-processor builds in Orcas and specifically how they work (unofficially) in VS2005, I had to try it out.  And it turns out that it works very well in VS2005, except when it fails completely.

You can parallelize all your VS2005 builds by setting a global environment variable CL=/MP2 (or /MP4 or however many parallel builds you want).  It seems to work great for a while – actually reducing the rebuild time for a significant project I work on from 15 minutes to 10 minutes – but sometimes, it seems it gets confused about dependencies and ends up rebuilding projects that don’t need to be rebuilt.

Is anyone using /MP with VS2005 successfully?  Did you encounter these problems, and how did you get around them?