On my system (a humble 2GHz MacBook with a single GB of RAM) Safari frequently outperforms Firefox in terms of speed and responsiveness. However, its RAM profile is appalling.
I got this after opening maybe five windows, most with just one tab, and one with about six or seven tabs, including two instances of Google Maps and one of GMail (which we're now apparently supposed to start calling Google Mail).
What prompted the memory profiling, ironically, was the Apple Store. Despite discovering the super-nifty new iPod Touch - an iPhone without the phone - my visit to the Apple Store site was not a resounding success, because it knocked Safari on its ass. The Apple Store window froze, and in the process froze up every other window, including several Web apps I was using.
The crazy part of all this is that I closed every Safari window a few minutes ago but left the app running just to see what it would do. So far, its memory consumption has lessened as a result of having no windows open at all by a paltry 11MB. That is to say, with several windows open, it was using 300MB of RAM, and after closing all of them it's now consuming 289MB of RAM.
Anyway, I don't really have any specific point here, except that Safari RAM consumption is completely insane.