I have the 708 and its spectacular. I will probably go to the 712 for the next pc, just because I really want to see the last of my PATA cables gone from my machine. Most pci-e motherboards will have 4 sata ports to run dvdrw, dual hd's and maybe one spare hd for backup.
If the 708 was a reasonable indicator, the 712 will be a keeper. Given I use DVD-RW's a great deal, the 16x drives that might be a touch faster than the 712, are really of no additional benefit to me. However, they will be a tiny bit faster. Personally Plextor has given me better products over the years than the others so I'll be taking a 712.
Hope that helps.