They purely aren't mainstream yet, they are still in their infancy as they are still trying to get better access times, price to gigabyte is shocking, they are purely for performance enthusiasts at the moment, until they become better priced per gigabyte.
Kingston SNV425-S2/64GB = $270
Kingston SNVP325-S2/64GB = $331
Intel SSDSA2MH160G2C1 = $933
Seagate 500gb = $85
WD 640GB 64mb Black = $121
WD Raptor 450gb 32mb = $448
WD Raptor 250gb 16mb = $240
As you can see, price to gigabyte, SSD's are not mainstream, UNLESS you are going for performance, then they are for enthusiasts.