Nothing is a fast a RAMdisk. But that's about the only advantage of a RAMdisk
A RAMdisk is usally small. FancyCache can easily work with several 3TB harddisks. To do so it only uses a few MB of cache.
So FancyCache uses, say, 50MB to improve the performance of 5TB harddisk space.
FancyCache is as your wrote a smart cache system.
It tries to minimize the number or read/write actions to HD.
It has little use if you read/write a big file to harddisk.
But quite often several programs are accessing your harddisk and/or small blocks are written to HD.
FancyCache tries to organize data requests in such a way the disks are accessed in such a way the harddisks work fastest.
Example: Writing 100 blocks of 4KB is much slower than writing 1 block of 400KB.
I think you should/could just try it and see if it works for you. It's no RAMdisk replacement so you can use both.
Things like this depend greatly on the speed of your harddisks and usage of your PC.
Just like a quad-core PC with a 100MB is useless for someone that only emails with his/her PC.
But for others it's worth every cent.
That's my 2 cents