Here's my .02 on the D908. (I have one on my 630)
Cost 2X as much but lasts 2-3X as long as a T63/D606/etc so maybe you save money (especially if you're paying someone to change tires). There's also value for someone setting out on a long trip and doesn't want the aggravation of planning a tire change 1/2 way to Alaska or the TAT.
It's a very stiff, strong, puncture resistant tire. Not getting a flat is worth a lot to me. With a Tubliss insert it's a run-flat.
Fast, aggressive riders report it doesn't offer great traction. Hasn't been an issue for me but I'm slow.....
Surprisingly it mounts very easily, much easier than the T63 I mounted the same day.
Here's a pic I grabbed from the Sibersky Extreme thread. This was after 9000 miles on a KTM690.