From what I've read in various places it's not actually all that great for overclocking due to not moving the heat away quickly enough. Good for a silent system maybe but extremely messy, apparently any cables that are dipped in the oil tend to wick the oil out of the case. 3M makes a submersive coolant liquid specifically for server applications but it is very pricy ($100s) and doesn't seem to fair much better than mineral oil.
There's a guy who posts here from time-to-time that has done some crazy stuff with a watercooling loop cooled by an A/C unit
. It's pretty damn impressive. For a while I was thinking about trying something like that with this old rig of mine but the costs and diminishing returns put me off, I'm not hardcore enough for that. Things really seem to level off fast after you get past the limits of watercooling (unless you have the means to run your system on LN2 24/7).
edit: fixed url