personally i dont like them.
A. the S3 gets so hot under the bonnet with the turbo, even more when chipped, Sucking in that hot air is a big no no.
Ok you can route a duct to the front bumper for a cold air feed, but the ammount of standing water ive seen in the middle of our summer, with these flash down pours would put me off.
I drove to work the other day, and must have been in over 3 inches of water for 15 miles!
Sucking in water through a cold air feed duct would also be a big no no.
B. most, unless you get a stainless ones contain oil, which could/can/still under debate... damage the MAF
The audi paper one is actually very good. I just change it every 10 K miles for £8, and its good for 275 bhp (rolling roaded)