I'm a little concerned about it effectively being a sealed unit, with only memory access being 'allowed' by Apple. I'm beginning to consider a switch to PowerMac early next year, but we'll see.
I've been happy with my iMac so far, but it'd be gone for weeks if I had to send it back to Apple for a repair. Shipping can be slow here.
My iMac is a little buzzy, but I wouldn't consider it to be a 'problem' - really, it is still quieter than the WIntel laptop I have. I am generally delighted with the iMac.
My imac's remote rarely works (despite being replaced by apple) and contacting them many times and the screen has a curved line on it made by a slight contrast discrepency (annoying but not too distracting).