Grease is your friend. After 2-3 long track days I pull the seals off, spray the bearings out with Nitrowash or similar, re-grease with Avid Slide grease and re-install the seals. I do this on all the drive train bearings on my MBX6r. It's really not alot of extra work and lets me know that the bearings are healthy, and smooth spinning. Haven't had to replace a bearing in a very long time. Of course I don't do this for the clutch bearings. I replace them after 2 track days.
I've heard alot of people complain about these bearings or those bearings being gritty, or notchy. Alot of that is just dirt in the bearing that gets pushed in by aircompressors or spray cleaners, or just lack of maintainence.
My 2c