They improve load times tremendously. RoF missions in 10-15 seconds, tops.
They are very reliable when set up properly (correctly aligned and with minimized number of random writes).
Thorough guides are available online.
Currently using this one myself:
http://www.ssdflashdrivereviews.com/flash-drive-review/Corsair-X64-64GB-Indilinx.phpExcellent value for money.
As an aside, would anyone be interested if I wrote a short Buyer's Guide for people wanting to upgrade their hardware for RoF?
I can offer some modest yet sensible advice for simmers on a small, mid-range or large(r) budget.