There's a great deal of difference between various aero wheelsets. Take a look at the current issue of Velo that compares eight.
As already mentioned above, rim shape is important. Companies like Zipp, Enve, and Hed spent a lot in designing and testing. Many generic rims have a deep shape and that's it. They might not do much at all - or could be great. You just don't know unless you see actual comparative test results.
Last edited by StanSeven; 09-22-14 at 07:39 PM.