It would be disorienting at it's best!
With that said, I actually like backward coasters. I love Sidewinder at Hershey. A number of coasters at Six Flags Discovery Kingdom in CA are great. And I miss when they use to have the Rebel Yell at Kings Dominion go backwards.
Dark coasters, on the other hand, I have no problem with, and even the most intense of them that I've experienced (Flight of Fear and Space Mountain: Mission 2) are rides I rode several times during my visits to the respective parks. I never once felt any ill effects. For thrill rides, I still strongly prefer outdoor rides where you can tell where you're going, but for relatively tame coasters being in the dark can add significantly to the experience (example: Disneyland's Space Mountain is a really good ride, but if the coaster was in the middle of an open field it would be a very boring ride).
I truly hope Great Adventure tries this experiment with its version of Batman (same as Great America's)...I'll be a willing lab rat.
If I was to try a backward ride, I think that Colossus at Magic Mountain during Halloween would be a different experience. They run it backwards, or at least they used to, I think the noise alone would be intimidating. Most of the backward rides are pretty much straight line or, like Boomerang, just redo what you have already experienced.