I've got a 20" and I was playing with the 27" in the apple store today... It's too friggin big! The 24" is almost too big, almost.
You would need to sit like 3' away.
Coachingguy
I have a 20" iMac and was looking at the 24/27's recently but decided to get another 20" monitor with good resolution since the 20" iMac still handles all we throw at it fine... now its a dual screen 40" at a great price