I used to have 24" iMac, upgraded to 27" as soon as it came out (late '09 model) and I LOVE the new size.
What you have to bear in mind, is that it's not so much the size which is the important factor here, it's the aspect ratio of the screen. The 24" iMac was 16:10, whereas the 27" is a 16:9 display.
If you put them side by side, you'll notice that they're almost the same size in terms on height. The new iMac get's its extra inches from being wider than the 24"
So really it comes down to what you prefer, a 16:10 vs 16:9 display.
Unless of course you're after a 24" 16:9 display! Personally, I don't think Apple would do that. Nor do I think theres a huge market for it.
We've got both the 24" and 27" at home and had 17" and 20" in the past. There was a small period of time (maybe a year) when iMacs were available in 17", 20", and 24" simultaneously, but in general Apple has never been about customer choice.