They work but given the 3 areas you deal with... your OS, storage of sound libraries, and storage of project files... I'm assuming in your going to put your libraries or your projects on a slower drive system that you would want that to be your project files. I know there's writing involved with your project files but that's not really a speed issue, you'd want the loading of libraries on the SSD right? It was a late night and my heads pretty foggy
My priorities (highest to lowest) for fastest storage:
1. Sample libraries;
2. Record drive -- place where audio gets recorded;
3. Project files (really can be HDDs)
4. Boot drive (C: drive on PC or Macintosh HD on Mac OS)
5. Synths (Omnisphere, for example) -- really can't discern any improvement with SSDs, but possibly there is one.
OS Drive with SSD? Opinions Vary
Some people write that their computer seems "snappier" with an SSD on the OS drive and, for full disclosure, I do have SSDs on nearly every computer now. That said, for me the improvement is either negligible or even imaginary, except for startup from being turned off, which is indeed noticeably faster.
I suspect one's experience of the SSD-OS drive depends on one's style of working. For me, the improvement is small, which may be because of what I write and how. I generally compose larger-scale orchestral or orchestral hybrid pieces for media and work with a template that I load at the beginning of the day and don't mess with too much. Accordingly, once I spend the time to customise the template for a new picture or show or game, I am not starting each track from scratch and, therefore, don't typically load lots of new software all day from the OS drive.
So, if you work the way I do, HDD is ok for boot.
Others, who seem to be more songwriters (guitar bass vocals drums in other words) seem to be the ones who laud the benefits of an SSD for the boot drive.