I've used both on a Mac. When Apple started offering both, I agonized but finally opted for glossy and haven't looked back. My preference on most things is matte - I always opted for matte when people printed photos from something called "film."

Here's why glossy screens are better, for me anyway:

1) It's better in sunlight. Counterintuitive I know, but it's far, far better. Matte screens diffract light and render the screen unreadable. Glossy screens can be made visible either passively (in very strong light) or actively if the screen is bright enough (as many now are.)

2) Glare is controllable and not diffused on a glossy screen. With a matte screen, glare manifests as blobs, and spread across the screen. With a glossy screen, glare is localized and easily defeated by repositioning the screen.

3) Matte makes screens less bright. It's a translucent filter, so of course it reflects and absorbs light.

4) Matte filters can be added, but a matte screen can't be made glossy.

The real solution: get rid of awful overhead florescent lighting. Ok, not that easy for most of you, but I think Steve Jobs probably wants those gone as much as he wants Flash gone. Any word on lighting at Apple?