They have several entries in the Top 500. After taking the two-day class MS put on for their HPC 08 product, I have to say that it's a decent product.
For a product that's as new as it is, it's fairly functional. There are a few functions it doesn't have, but since it's built on top of Server 2008, it's pretty stable.
It's biggest hurdle it must overcome is the application base. Cluster apps tend to run in Linux/Unix. There aren't a lot of Windows-based parallel processing apps out there. They are really pushing for the smaller sized, department-level clusters. That market is growing the fastest at this point.
You missed the sarcasm. Since free Linux made supercomputers for the Chicoms possible, Microsoft is now making it point and click easy. This technology should have never been openly released in the first place, and should be export controlled now.