I have done it once, but I understand that each year could be completely different. More than any other place, you are at the mercy of Mother Nature.
When I rode it, it started with light rain that lasted for a couple of hours, and there was a persistent headwind in the second half. Bonnie Claire flats towards Scotty's Junction was fun, easy 25mph with barely any effort. Coming back required a lot of effort and teamwork, barely making 12mph in a paceline. And the easy looking mostly downhill section from Grapevine to Mud Canyon required some effort.
The support is good, especially considering the remoteness of the place. Which bike you choose is up to you, as I don't know if you are used to riding such long distances on a tri bike. Just know that wind could be a factor. I did not need a Camelbak, rest stops are well placed and stocked. It is a special place, as Marshal Ulrich said, "expect a journey and an adventure".
I'm signed up for it this year and am hoping for favorable weather. I had planned to ride it in spring 2011, however the weather forecast had been for considerable rain in the valley, and I ended up staying home and DNS'ing. It turned out only 10 riders finished the double that year, and if I wasn't even willing to drive/camp in that weather, there was no way I would have finished it.
In my searches of double century ride reports, Death Valley Spring and Fall probably had the most written, so you might try Google as well.
There is a lot of useful information on his website. DV Double is good support and a great experience. "MetinUz" is right, each year is different and you need to be prepared for Mother Nature. But all Adventure Corp events are very well run and well worth the trip!
And road bike vs tri - whichever one you are able to spend that distance on comfortably.