I have a 300+ project solution that we benchmarked on SSD and spinning rust. SSD saved us about 10 seconds of a 200 second compile. The bottle neck in our solution was the Silverlight projects, nothing we could do could make them compile faster! Even tried RAM disks after that and the compile time didn't drop noticeably. This was on a top of the range i7 with buckets of RAM and SSD for OS.
The biggest win for us though was in opening solution and source control operations, made a real difference in usability there.