i'm currently using a 21" iMac and I've just had my 27" display (which acted as a second desktop) taken away. I'll have to side with the larger 27" display. I used to have a MBP at 15" and then added this as a secondary monitor. and I loved it. the rezolution is by far teh best and the additional TB hookup+built in powercord for my MBP meant I kept my powercable always in my bag and never "left without it"