We've been using SSDs in our servers since late 2008, starting with Fusion-io ioDrives and Intel drives since then - X25-E and X25-M, then 320, 520 and 710, and now planning to deploy a stack of S3700 and S3500 drives. Our main cluster of 10 servers has 24 SSDs each, we have another 40 drives on a dedicated search server, and smaller numbers elsewhere.
What we've found:
* Read performance is consistently brilliant. There's simply no going back.
* Random write performance on the 710 series is not great (compared to the SLC-based X25-E or ioDrives), and sustained random write performance on the mainstream drives isn't great either, but a single drive can still outperform a RAID-10 array of 15k rpm disks. The S3700 looks much better, but we haven't deployed them yet.
* SSDs can and do die without warning. One moment 100% good, next moment completely non-functional. Always use RAID if you love your data. (1, 10, 5, or 6, depending on your application.)
* Unlike disks, RAID-5 or 50 works pretty well for database workloads.
* We have noted the leading edge of the bathtub curve (infant mortality), but so far, no trailing edge as older drives start to wear out. Once in place, they just keep humming along.
* That said, we do match drives to workloads - SLC or enterprise MLC for random write loads (InnoDB, MongoDB) and MLC for sequential write/random read loads (TokuDB, CouchDB, Cassandra).