I think its great for mastering. Yes it is a hog, plus it has over 5000 samples of latency(and its not a plugin I would use on a individual track). The new version 8 does a damn good job really quickly and its rapidly replacing my usual chain
If you are worried about horsepower, its another good reason to leave "mastering" separated from mixing
My thoughts on the 4GB Ram limitation: I understand on paper, that anything above 4GB(actually around 3.4GB) is wasted on a 32 bit app(like Pro Tools 10), if you are running a 64 bit OS, you may still benefit from extra RAM. My empirical evidence relates to my own move from XP(32 bit) to Windows 7 64 bit. On that computer(i7 quad/X58 mobo/6GB triple-channel RAM) with no hardware change at all(from XP to Win 7), my Dverb 2 test count went from 320(on XP) to 500(on Win7 x64). Just food for thought...