Keep in mind, or course, your mileage may vary. Instead of buying a large, expensive, inflexible mainframe, you buy a bunch of small, cheapo, configurable wintel boxes. Instant cost reduction. Instant mix and match flexibility. However there is a potential weak link in the Microsoft LCC story.
Clusters are a lot more complicated to manage than single machines.
Somebody needs to make sure that the machines are all appropriately configured, that correct versions of the application are installed, that the machines are all working, and that the work load is being evenly distributed. To put it simply, in order to make an LCC usable, you need something that can do for the machines in the LCC exactly what Mike does for the baristas at our Starbucks. Make sure that they are properly configured, updated, and well managed. In the past, Sun has criticized Microsoft's LCC strategy. You are much better off using a single big box.