I have seen many posts about WD Velociraptors. Upon upgrading my i7 920 from a VR 300GB to an X25-M. Obviously, the performance difference was very noticable.
Out of curiosity I doubled up two velociraptors 300GB into a Core 9650 (RAID 0) to compare performance.
Sequential reads -- copying multi-gigabyte files in a synthetic test (I don't copy may 10GB files during normal use) -- was pretty comparable. However, the RAID 0 velociraptors were not noticably faster than the single VR's on random reads, which just affects so many things (like starting up big programs).
The noise and heat difference was totally disproportionate too. On the i920, I'm able to use a Small form factor case (using a Rampage II Gene). I'm able to cool with just the CPU cooler and two low RPM 120mm fans in the front (meaning, very quiet).
On the 9650, I can use a SFF motherboard, but 3 hard drives (two velociraptors and 1 storage drive) just generate too much heat to use a SFF chassis without getting silly with fans.
The major deficiency of the SSDs, IMHO, is that 160GB is a pain in the neck. It's just too small for a single-drive desktop system.