I've recently been trialling CloneDVD which supports backing up DVD -/+ R/RW, loosing CSS and region coding in the process. My PC will burn and playback in all formats and I've played back ups on single layer DVD-R's on a domestic machine with no discernible loss of quality as a result of compression.
As far as I can see currently the only double layer blanks available are DVD+R. I've backed up a commercial DVD using Verbatim (£29.99/5 at PC World) and the copying went fine and plays OK on the PC.
It will not however play on any of our domestic players. I've logged a complaint with Sony as their manual does say our particular main domestic player will playback all formats.
I think double layer is all pretty new and might be a bit hit or miss as yet. Backing up to compressed single disks or uncompressed via 2 discs does seem to be a bit more settled.
I'd be interested to hear how anyone else has gone on.