Conclusion
Although double layer rewritable DVD drives are generally more expensive than their single-layer siblings, their additional capacity and ability to write to single-layer media at the highest possible speeds makes them a useful alternative if you need more than 4.7GB per DVD disc.
For Further Research
DVD+R DL Technology
Philips, one of the founders of the DVD+RW Alliance, has a useful discussion of double layer media, including a diagram of the double layer writing process and the official DVD+RW DL logo at www.dvdrwalliance.org/press/doublelayer.htm. See the DVD+RW Alliance website at www.dvdrw.org for additional background information.
Firmware Upgrades
The Beginner's Corner DVD Media www.supermediastore.com/beginner-in-dvd-corner.html offers additional links to firmware updates for major brands of rewritable DVD drives.
Drive Reviews and Benchmarks
You can see some of the best reviews of double layer drives at the CDRinfo.com site: