I would tend to agree with Bit on this one...
For streaming samples (e.g. Kontakt) they will add some life to an older machine.
For OS, and especially for temp files they can also be a real machine stretched.
For my project files? I'm inclined to stick to spinning platters - they have a longer life span, they are less expensive, and they can provide a warning before going belly up. They don't always do that, but at least they can. And I don't need the speed of SSD for audio files - yet. I suppose when I switch to 64 bit at 192kHz I might have to reconsider. But by then SSDs will be a lot cheaper<G>!
For now I can get away with a 240GB drive for the OS, apps, and temp, which also reduces the heat from two drives (I still keep all temp directories on a separate drive - not sure it makes a difference, but it is habit.).
I need at least 1TB for streaming libraries, although I could put most of the larger libs on a 500GB drive and leave a copy of them, plus all the smaller libraries and SFX on the current 2TB drive.
Man I need more money<G>...