I remain convinced that the 21.5 inch iMacs have glass screens, even though, while tapping on my 24" right now, I agree that the resonance does sound "plasticky."
The IPS display on the 27" is riveting, and the LED lighting makes the color contrasts all-the-more vivid.
Until OLED gains the ability to produce greater pixel illumination, I see the IPS LCD remaining desirable for the next few years.
I have a matte screen 30" ACD which I love - however, the 27" iMac screen seems slightly more crisp, vivid, and provides more color contrast and saturation.
Matte screen covers are available for those who prefer matte - i personally appreciate the clarity of glossy, as reflections have not been a hinderance at all, while using the glossy iMac.
To each, his/her own.
Do you do any photo editing? What is your lighting situation and how are the reflections? Did you calibrate the monitor? Thanks.