judging from some other people's posts, i would say 4x is about average. the key is how well was the movie ripped, and if your media is good. Here's what I do:
-I always try to buy the best DVD-R media (Verbatim, Taiyo Yuden, Ritek)
-I rip the movie with DVD Shrink and use adaptive error compensations and deep analysis. (don't compress too much, use DVD Decrypter when needed)
-If I downloaded the movie I use 4x, if it's a retail dvd I ripped I use 8x.
-I have a Lite-On 1633-s (or something like that) I don't think it's great, but I don't think it's crappy either.
using good media (Taiyo Yuden and or Verbatim) amd a good machine ,burners,memory etc. you can burn at 8x as long as the media is rated 8x , most in here go 4x max. though ,using good blank media is the trick