I've had no issues with bearings in any of my Xrays and I've owned all versions of the 808. I had the original 08, 09, still have a 10 and just got an 11. I also have 2 XT8s with no bearing issues.
I know my friend goes through bearings on his Serpent faster than I do. I've seen his wheel bearings create quite a bit of slop fairly quickly and that's with Avid bearings. I use Avids too and don't have them get sloppy on me like that, I can run them for 2 or 3 months before needing to change.

I think there's a couple of reasons for this though. He runs the Serpent aluminum hubs all the way around so much more force is transferred directly to the bearings when he hits something due to the lack of flex. He's also a first year racer and was hitting the pipe and wrecking a lot until lately. His driving has really started to come around.
Another issue with the aluminum hubs is that Serpent doesn't use any sort of sleeve for the bearings. You have aluminum mated directly with the steel surface of the bearing and they are very hard to get out as a result. Xray uses a thin plastic sleeve over the bearing for their aluminum hubs. This prevents the issues created from 2 dissimilar metals being in direct contact with each other.