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Glossy is not better for image quality ( btw, no idea why the mobile version of macrumors doesn't do qoutes or replies correctly )
Pro high end monitors by Eizo for professionals who do movie and picture editing really hate displays with glare.
...and yet the best image quality HDTVs out there are plasmas, which have a "glossy" type display...
Note that "glossy" is not a display technology, like LCD, OLCD, Plasma, etc., thus each case may differ, as to whether or not a glossy version looks better than a anti-glare version.
The glossy display in the mbp allows for a bit more pop in the colors and darker blacks, etc., which appears as better to the human eye - which most refer to as better "image quality", while the anti-glare is more color correct in relation to print, so you can say that the anti-glare is better image quality for on-print.
Professionals don't particularly like glare for work because they have to look at the screen and see every minute detail in various environments, so naturally the harder it is to see the screen, the harder their job becomes - this has nothing to do with image quality, just convenience, i.e. location of screen with regards to light sources. In fact, I have an anti-glare screen at work, but I also have a large window directly behind me, so it's fine with me (and yes, I can and have used my glossy mbp at work without any problems, but I've never had issues with screen glare like some people do - doesn't mean anything against anyone, glare just bothers some and doesn't bother others).
This isn't rocket science, anti-glare screens are well known to wash out colors a bit, but of course, at the benefit of reduced glare from external light sources. It's a trade-off. If you're in a well lit area, anti-glare will look better and if you're not in a well lit area, "glossy" (non anti-glare) will look better. Pretty simple, though many in this thread blow the glare effect on the mbp's glossy screen a bit out of proportion, and I felt the need to speak up for the glossy's image quality performance, as it was not being accurately portrayed.