Frequent write is not good for NAND type SSDs. Not only are they slow but wear out will be faster too. So they are not good for a cache drive. Drive controllers mask this slow write by a DRAM write buffer. But in the end data must still be written to flash. Erase operation is even slower. Erase affects SSD operation the most when the SSD is almost full, then it would almost grinds to a halt. HDD has similar problem when full but they are usually much larger size.
Because read speed is very fast, Window7 should turn off pre-fetch; only if Windows recognize the drive is a SSD correctly.
I never turn off my computer, only use sleep mode. So SSD doesn't mean much for me. I use them with carry around notebooks because they are much more rugged.