yes, it cut down the drag and decreased warmup times. realize that I covered it in such a way that it could be reversed before actually molding it in. after 1900 posts, it would take a while to find the first time I talked about it.
here is a video of me molding my bumper. realize that I am not a paint/body man so some of my practices are probably less than acceptable.
http://www.youtube.com/watch?v=4n6iNOxl5PA
there are also pictures in my garage.
I would say to do it with coroplast or plexiglass before hand to make sure it doesn't jack with your cooling system. mine was covered for I think 2 years before I did this. my grill has been covered almost the entire time I have been on here and it has been close to 4 years for me. no overheating issues yet.