1) Macs work fine for games. Just as in the PC world, game performance is based on hardware AND software. A badly ported Mac version of a PC game will obviously have performance issues when compared to the PC version running on a similarly equipped PC.
2) As someone already mentioned, the Mac version is being worked on. The Mac version was announced at the same time as the PC version. For those of you who don't know, the first major public viewing of Halo was at the Macworld New York 1999 keynote speech. It was running on a Mac. Last summer, the Mac and PC versions were reannounced. Of course, this new version of Halo is completely different than the one shown at MWNY 99. Instead of a 3rd person multiplayer focused game, it's a 1st person singleplayer focused game. The PC version ported by Gearbox, the Mac version being handled by Westlake Interactive, the best Mac porters in the business. There work is based on the PC code. There's video footage of an alpha build of Mac Halo running on a G4 867 with a Radeon 9800 Pro at MacGamer. The Mac version is now in Beta and should be ready to ship in time for the holidays (my estimated shipping date).