I played with one at the Apple Store last night. I can not stress what a gorgeous machine this is. The LED screens are amazing. Made the 30" ACD look washed out.
They don't have an quad cores yet but the C2D was most definitely snappy™.
I played with a new 27" iMac 3.06GHz model last night (they salesperson actually unpacked their first demo model and set it up while I was there).
It is a great machine, but we did run a quick XBench test against the older 24" iMac 3.06GHz and the new model model faired a little worse in some tests and better in others (I can't remember any of the actual figures now). Not quite sure why though.
The 27" iMacs are quite weighty, so you'll need a sturdy desk to put them on.
I never really had a problem with the Mighty Mouse (apart from always switching off the extra buttons), but the Magic Mouse feels better in use and works great, with the possible exception of the two-finger gesture. The Magic Mouse is also thankfully less weighty than the heavy wireless Mighty Mouse.
Edit: Looking at the topic title I think I've discovered why you're having problems with the screen ... somehow you managed to get a 27 foot iMac instead of 27 inches.