I recently replaced the SAS drives in all of my database servers with SSDs. Watching IO wait on my usage graphs drop while db latency fell to near-0ms was such an incredibly gratifying feeling. Worth every penny.

If you've got heavy random IO on your dataset (90% active, OLTP here) and/or deal with any sort of significant contention or deadlocking across your disks, I would strongly encourage upgrading to SSDs. Of course, if you can fit your dataset into RAM, go that route. But if you're well past that point, might be time to consider SSDs.[1]

[1] http://www.mysqlperformanceblog.com/2010/04/08/fast-ssd-or-m...