IMO, it doesnt matter what speed you burn a single layer (DL is a completely different story) disc at these days, assuming one uses decent quality discs.
Ive just finished burning 20 single layer DVDs at maximum speed (12x) and didnt have a single fail using verbatim discs.
I strongly believe andrews problem relates to the DVD recorder his client is using, rather than the media hes created the DVDs with.
we have near on 15 DVD players of varying brands and age in our office and its well established that some of the older ones simply will NOT play authored DVDs from encore, nero, or .. well, anything. Yet i can go and put these same discs in any of the computers or other dvd players and they work fine.
To put it simply, if the disc burns successfully, and it plays in 3 out of 4 different DVD drives/players. Then your disc is fine.
~Peter Berthet
Sydney, Australia