Before I post some test results on Dual Layer burning I will attempt to explain a few basics
Dual layer as it states is two burning layers on a DVD+R recordable disc and will increase the capacity by nearly double it cannot equal double a single layer disc because of overheads required with this format and the disc sizes is increased from 4.7gig to 8.5gig The maximum capacity of a single layer disc works out at 4.38g and for dual layer approx 8g. The dual layer disc has to be compatible with the standard DVD player and I have found that only the very latest players manufactured during the last 12 months are compatible with these discs and earlier players either suffer from erratic playback or do not play at all, as the media is further developed I expect better compatibility.

For the DVD+R D/L disc to be compatible with domestic players I have found it needs to be burnt with the DVD-ROM book format instead of DVD+R book, and I find that in most cases this done in the firmware as standard for DVD+R D/L media or bitsetting can be applied prior to burning, this I feel can be a problem if not set to DVD-ROM book type would result in an unplayable disc.

For the discs to play back correctly both layers have to cover the same area of the disc. The layer change point is set in the burning software at approximately mid point. With the domestic burning software it does not appear possible to set this point manually so the switch point is not always at a desirable point. The layer switch causes a slight pause in the playback and will jump 1-2 seconds at the switch and I think this is something we will have to live with. Dual Layer media is not only for Video it can equally be used to store 8gigs of data.

Not all burning software will support Dual Layer burning and only the very latest versions are likely to be suitable. I have successfully used the latest versions of NERO, RECORDNOW & COPY2DVD to produce playable movies.
The current burning speeds for D/L media is 2.4x and 4x although very few drives support 4x and 4x media does not seem to be available yet. It takes 40-45 minutes to burn full disc at 2.4x and there is little difference in the burning times with the drives I am testing.

It is not possible to do direct backups of your DVD-VIDEO collection onto Dual Layer media because of the copy protection built into the disc.