Verbatim is the best brand for DL's I think. I've so far burned 9 DL's without any issues from them. I've heard bad things about the Memorex's, but thats hearsay.
True, they're rated at 2.4x, so a full DL takes about 40ish minutes to do. If you have a faster DL burner, the verbatims should be able to be pushed a little bit to go a little faster.
DL produces the full DVD quality movie, just like you buy in the store. Typically all movie DVDs in the stores are DL. If you want a movie-quality DVD, it only fits 1hr on a DVD-/+R. Of course you can always play with the bitrates to get it to nicely fit on a DVD-/+R or a DL.