A few generalizations to spark a few more questions:
Different drives handle different models of media better than other drives, and some drives are known to handle DL discs better than other drives.
Generally, Verbatim/MKM DL disc models are most compatible, in comparison to Ritek, CMC, and Ricoh disc models.
Ritek usually makes its own disc models, which aren't great, but can be OK with the right drive. Ricoh DL disc models made by certain manufacturers are okay, but if made by Ritek, they can be equal to the normal Ritek media. There also exist fake Ricoh discs that are of a lower quality than these, even (using only the Ricoh disc model names to be "more compatible")
CMC's own disc models are usually regarded as being of a lower quality than Verbatim, also. But when CMC produces media for Verbatim using Verbatim/MKM technology, the discs usually come out with a decent quality.
Older Pioneer drives (before and up to the DVR-116 or DVR-216) seem to do really well with DL media, as do a number of Optiarc models. Some LiteOn drives handle lower quality media well. It truly varies.
Write speed may also come into play here. Writing at a little under the maximum speed sometimes helps if you have writing errors. But this cannot make up for bad media.
[edit: typed before I knew Kerry was going to post above me. His words are wise. :iagree:]