TrustedReviews has posted a review of NEC's new dual format, dual layer recordable drive, the ND-2510A. Almost identical to their current drive (the ND-2500A), the drive can write DVD+R DL media at 2.4x. So how does the drive perform? Here's a snippet from their review:

I burned 7.9GB of data to the dual layer disc, made up of a single MPEG2 file and a DVD ISO image. It took 41 minutes and eight seconds to burn the whole 7.9GB to DVD+R DL, which is obviously a lot longer than burning two 4.7GB discs, but dual layer is more about convenience than speed.

The files copied to the disc without incident and were accessible using the ND2510A. However, when I tried reading the disc on a number of other DVD drives, things weren’t so rosy. Inserting the DL disc in a Samsung SD-616Q DVD-ROM, an Iomega DVD writer, a Sony DRU18A DVD writer and a Sony DDU-1612 DVD-ROM drive all resulted in failure. None of the drives could see the disc, let alone any files on it. However, when I loaded the disc into a second Sony DDU-1612 DVD-ROM drive it worked perfectly. With this in mind, I flashed the latest firmware into the other Sony DVD-ROM drive, and sure enough it read the dual layer disc without any problems.

So it looks like there could be some compatibility issues with older drives, but a good old firmware flash could sort it all out. Obviously newer drives will be produced with support for the dual layer format from the outset.