Ars Technica picked up a report on a study on Windows Vista performance that was conducted at Principled Technologies, a technology assessment company in North Carolina. There's lots of good data in the study about the performance of Windows Vista on a handful of different hardware platforms. One of the highlights for me was reading that they observed very little impact on responsiveness with Aero enabled.

They indicate in their report that of the 192 measurements conducted, only 4 were a tenth of a second slower w/ Aero enabled, only 2 were slower than half a second, and handful of tests yielded greater responsiveness with Aero enabled.