(2010 update: Of course, these days we can run Portal (and Steam, TF2, ...) on Mac. And the visual glitches mentioned have all been fixed in newer versions of Portal and CrossOver, too.)( Glitch and workaround... )
Portal is great.
I especially liked the moments of stare at layout baffled ... OH! ... portal-portal-portal-ding!
A bit of advice: Don't go on YouTube and watch a bunch of videos. There are only 19 levels (25 if you count the advanced maps, which are modifications of 6 of the regular ones), and having several of them spoiled was a bit disappointing.
I haven't tried any third-party maps. (Got recommendations?)
Oh, and one tip: when bouncing through a pair of floor portals and trying to aim at something, be looking straight up or down when you pass through the portal — that way, the game won't reorient you while you're trying to aim.