"With a comment like that, I'd be willing to bet you've never even used a Mac."
If they then say, "Yes I have."
Then probe to find out when it was. Most PC users I've run in to have actually used a Mac, but it was back in 1995 or somesuch. I've been using Macs since 1987 and 1995 was not a pretty year for Apple, much less Mac OS.
Comments like that are best dealt with by demonstrations of why Mac OS is clearly better. I for one carry my Titanium/500/512/mb/20GB everywhere I go.
If I get in to that conversation, I open up 6 QuickTime movies and play them all... with sound.
Or pop in The Matrix and play MP3s while watching the movie and working in my email program.
Or bust out a game of Unreal Tournament/Tomb Raider Chronicles/Quake III Arena with my own iTunes soundtrack. Better than that is to be on the internet and stream it.
Think of it this way: Mac OS X.1 is what Windows XP wants to be without all of the fluff and bloat that Microsoft is famous for.
"If everyone else jumped off a bridge, apparently you would too." "The best version of Office actually runs on Mac OS X, wanna see?"
Oh yeah, one more thing... if they say Mac OS sucks, you can proudly say: "Yeah, Mac OS 9 sucks for sure. But OS X KICKS ASS!"