At work we have a new 17" Studio Display attached to a Power Mac, and at home I have the 17" iMac. Although the horizontal resolution is higher on the iMac, I'd have to say the 17" stand alone display is superior.
I have been a little disappointed in the limited viewing angle of the iMac. Additionally, the screen is noticeably, but not drastically, darker at the top than on the bottom. After purchasing the iMac, I went in and looked critically at the Studio Display to see if the shortcomings existed there and maybe I hadn't seen them before. That was not the case.
That being said, I am very happy with the iMac. My computer is in my living room, and its level of quietness, the wider screen, and the cost for performance outweigh the lower screen quality.
John