I think Aqua is a little more functional than Aero. Take Expose. In one push of a button, OS X shows ALL of your windows at once, so you can quickly and easily find the one you want. Vista has a new feature similar to this, which I think is called QuickFlip3D or something. While the look of it is definitely cooler, it really only shows one window at a time, and you have to cycle through a lot of windows before you find the one you want. That definitely takes more time than Expose, which shows ALL of them all at once.
I've used Mac OS 10.0 not too long ago, and Aqua was rather sluggish during that. Aqua has come a long way, but it's a rather inconsisten mix between the original pinstriping, Safari's brushed metal, and Mail and iTunes' aluminum look. I think Apple needs to be slightly more consistent with Aqua. I wouldn't mind seeing nothing but aluminum, and maybe they could bring back the semi-transparent window title bar from OS 10.0 and 10.1 back.