I have an internal SSD running Mountain Lion and my apps and an internal 5400 rpm drive with most of my other data (pictures, movies, music). I've put my Parallels (using version 7) virtual machine files on my non-SSD drive so that Mac OS is running off the SSD and Parallels is running on the non-SSD. I use Parallels primarily for development using Visual Studio and I was just wondering if it would be better to put my virtual machine files on the SSD or if that would slow down both the Mac OS and the virtual machine?
I put it on my SSD, like all other apps. I think the advice to put it on a separate drives is for when you only have hard drives - 5400rpm at that (slooow). With Visual Studio being so slow (been using it for years), it can certainly use the speed boost. Mind you, I just set up my iMac on Friday and set up Parallels this morning, but so far, so good. Windows apps are running very fast, and so are Mac apps.
In general, it's best to place the operating system and all the apps that you typically use on the SSD in order to gain the most performance out of the SSD.