SSDs provide a distinct advantage over traditional platter-based hard drives because they have no moving parts. This not only makes them more reliable, but increases their speeds. SSDs are up to five times faster than normal drives.
Of course, this performance boost comes with a higher price tag. SSDs are much more expensive than other drives and hold very little– up to 80 GB on some models compared to 2 TB for a typical desktop drive.
I think SSDs will eventually dominate the server hard drive market, but as things stand now, they are too expensive and too small to be economically efficient.