I'll pitch in my two cents worth about SSD drives.
What you get from using an SSD drive is speed: there's no seek time, no rotational latency, no need for a buffer.
What you give up is size (and money, but that's another story).
The whole issue therefore derives from the question "When does speed count"? Like any decent OS

, Windows has a memory manager capable of keeping frequently used pages in memory. When a program is running, the most frequently accessed pages are the program's code. Unless the system's memory is really overcommitted, running code should never be removed from memory.
What
does move in and out is data. Some of this is data stored within the program itself. The memory manager typically has no idea how (in what pattern) internal data structures are going to be accessed. It tries to keep them all in memory, if possible, and if not then it (probably) prefers to keep the most recently used pages in memory.
That leaves the data external to the program: files. Some amount of memory will be reserved for file buffering. (Note that one of the settings for Windows is whether or not the system is primarily a server. That changes how this is handled.) Various operating systems use different algorithms (sometimes multiple algorithms) to optimized file access.
If the system decides that you are sequentially reading a file, it will pre-fetch stuff so that your program's appetite will be satisfied.If you are writing a file there's nothing for it but to follow along, using delayed physical write operations so that your program doesn't have to wait.If you are randomly reading a file, then there's no good way the system can pre-fetch your data so that is likely to be the slowest operation of all.
Getting back to SSD drives, you want to use them where their speed will do the most good. That means you want to attack your biggest bottleneck: random access to data files. That's why, IMNSHO, you don't want to use an SSD for your C: drive unless it is big enough to hold everything. If it isn't, use it as the place to keep your data files
and the Windows virtual memory file. That might be rather restrictive, but it will give you the best bang for the buck.
As I've mentioned before, you can use symbolic links to relocate any folders you like without your programs knowing or caring.
There's one further point: I don't think file access is the real bottleneck for PD. I think it is generally the CPU. If anyone has done any tests, I'd be interested in hearing about them.
Jerry Schwartz