Since the 2012 iMac came out, I have lamented the lack of support for Target Display Mode on the Thunderbolt-based Macs. Before owning an iMac, I used to run a 27 inch Eizo monitor off a W7 PC and when working from home I would hook my work PC into the Eizo as well, so I had plenty of desktop real estate. For the past year or so, this has not been an option and I've been working off a crappy laptop screen when doing work stuff, with my iMac confined to personal use only. I had tried remote desktop software in the past to enable display of my laptop on the iMac, but had given up because it was too laggy, and various glitches made it an unpleasant experience for any prolonged use. For some reason, I just decided to give MS Remote Desktop client another go on the iMac. All I can say is WOW! What have I been missing? It now works wonderfully! I get full screen 2560x1440 screen display of my laptop and it is snappy and responsive with no perceptable lag. It's fantastic. I can easliy work like this all day long. I am just kicking myself I didn't persevere with this and get it working properly sooner. I post this for any other souls in the same boat as me. You CAN use your iMac as a 2560x1440 monitor and it's quick, easy and works very well indeed!