Everything I've heard is pretty much what Ken's said. These things are cheap junk. Very thin sheet metal that will permanently leak the first time you overtorque the valve cover bolts (and unfortunately that's very easy to do).
I recommend buying the heavier aluminum covers. You can buy Clifford, Offenhauser or Mercury Marine rocker covers. The Offenhauser has the nicest finish, the Mercury Marine covers are no longer made but can be found relatively cheap on Ebay (30-40 bucks, compared to $80-$100 for a used Clifford or Offy).
I have one on my 292 and it's great. They're larger than the stock cover and actually makes your engine look a little larger. Good thing about the Mercruiser cover is that they're forgiving if you slightly overtorque the cover, the flange won't bend, and leaks will be history.
-magic mike-
