I would like more screen real estate. I have a 24" @ 1920 x 1200 and do not enough room for a second monitor.
Unfortunatly most of the reasonably priced 27" + monitors and TVs do not go above this resolution so I would not get any more on the screen, just the same but bigger. If you work a long way from the screen or have poor eyesight this may be an advantage but in the small space I have it would not help.
I am really after more vertical space, first, and more horizontal second, a 27" 2560 x 1440 monitor would be ideal but I am not prepaired to pay the asking price at this momen