I really can't stand the glossy screen when it's large.
I suspect this is always going to be a personal preference thing - it also depends a bit on your environment and type of work.
The advantage of glossy screens is that they reflect most of the incident light away from you, whereas matte screens scatter it in all directions, so some of it will always be visible. Provided you can arrange things so that your screen isn't reflecting a light or window directly at you, it should be better than matte.
I work with my screen at about 45 deg to a window - when it was sunny, my previous matte screen was completely washed out whereas the 27" Cinema display is still usable in those circumstances. I find it easier to ignore reflections (you can 'focus them out') than to work around bits of the screen that you simply can't see. They might, however, be more noticable if you were doing phot editing, with large areas of dark colours.
Of course, if you're trying to do serious, colour-accurate graphics work with a bright light or sunlit window shining on the screen then
you're doing it wrong regardless of how shiny your screen is. There's a reason why pro displays (often costing 2x as much as the 'expensive' Apple display) have hoods.
Personally, the dealbreaker for me with the ACD/ATD is that it only has a single input, which is DisplayPort-only (ACD) or Thunderbolt-only (ATD) - I use an ACD at work where that isn't an issue, but i've not succumbed to one at home, where I have various other systems with DVI, VGA, HDMI that I want to plug in from time-to-time.
Otherwise, its the nicest display I've ever used.
(I'd also hold fire just in case it get updated when the new iMacs come out - retina is unlikely, but it might at least get USB3).