I prefer to use
ddrescue to rip DVDs for a couple of reasons: it skips over bad blocks, and it gives me good output of the read / write speeds.
Here is a sample of ripping a DVD. Note that the arguments are required for when it comes to DVDs.
ddrescue -b 2048 -n /dev/dvd dvd.iso
ddrescue can also resume a rip if you keep a logfile. The filename is passed as the third argument.
ddrescue -b 2048 -n /dev/dvd dvd.iso dvd.log
When ripping some DVDs, there is a chance it will skip over some blocks it can't access. In my case, so far, I've only seen it skip over one or two sectors. If it's doing more than that, it's probably having trouble accessing the drive in general.