On Macs, always play with Light Flares on Low or Medium. When set to High or Extreme, the game will lag heavily, particularly in areas where there's a lot of lights. Right now, apparently the Light Flares are done in software because Apple doesn't have the necessary extension(s) available in OpenGL.
Technically, Vertex Shaders should be much faster than No Shaders when the option is available and you have the bandwidth to keep the card filled. In Halo, The Vertex Shaders Only option will only show up if a card that supports it is detected, so you don' t have to worry about the sw emulated VS being used. But for whatever reason, the Radeon 8500 and 9000 cards seem to be performing much slower than expected on the few systems I've seen performance numbers for. My brother has a Radeon 8500 in his 1.4 GHz upgraded Sawtooth and he has to play with No Shaders. This may be due to the 2xAGP port and the 100 MHz system bus. Since Halo is so much more GPU dependant, it's important to keep the card filled, so for once, the AGP bandwidth actually matters a bit.
There are plenty of good games for the Mac. Most of the big name games get ported. Sure, we miss out on some of the big ones, but we still have our fare share of big name ports. More than enough to fill my free time.