I suppose SSD is good for serving static and dynamic files. With databases DRAM-Based SSDs are very good while flash based SSD's are quite bad (see http://www.storagesearch.com/solidda...omparisons.pdf) . This is because flash based SSD's have trouble doing random r/w. SSD has also longevity concerns with 24/7 writes.
At the moment I recommend 15K rpm SAS-SCSI disks for databases. SSD industry is moving fast, so it might not be long that SSD is viable option (and affordable) as db disk.