I have a New Beetle rear wing I got from the Real Source on my car. No apparent cooling problems. Mounted (with NO modifications to wing or car) in the same place as the Jsports one, but does not block off the vents like the Jsports one seems to do. Here's a couple of pics to show you what I mean.
I like the way it looks. Not too flashy, and definitly different. As far as function goes, I imagine that if I could get my bug up to high enough speeds, it would profide either some downforce or break up the airflow over the car so it wouldn't take off.