I'm actually quite surprised to hear this, because I have heard collectors complain regularly about DVD-R's that eventually stop playing, or that freeze, or that act all finicky, with a lot of hiccups. By all accounts the consensus has been that this type of media has yet to really be perfected...
I couldn't agree more. DVD-R's are not dependable, and this is a shame. We like to transfer our VHS tapes over to DVD-R for convenience, but the unfortunate irony is that we must still hold onto the original cassette tapes as the "masters", because that shiny little disc will never last as long...