The Atlas 10K II and the Cheetah 18XL perform very similarly for 10K drives,a nd cost roughly the same (I think the 18XL could be had for cheaper).
But for those who would love a Cheetah X15, Onvia has them in stock for $455, and with all the Onvia hot deals that are usually around you can get that price down I'm sure. Hell I may have to make the X15 my next purchase, I feel wimpy only having 9.1 gigs of space and only 2.5 gigs free.