I've got a rig and absolutely love the bike itself, but have had a bad experience with the EBB. One day when it was wet and muddy it slid laterally (side to side) on me without me knowing it. And then all of a sudden I feel my right crank arm start hitting my chainstay as if the crank arm had bent. I pull over, look down and see that the EBB had slid from the right (drive side) to the left in its shell. This caused the chainring to cut into the chainstay just like a buzzsaw (without me realizing it). There is no snap ring or ridge on the shell to prevent the EBB (and chainring) from sliding into the chainstay. Now the chainstay has a nice slot in a max stress area, just a matter of time before it breaks I guess (this is a nearly new bike too).
And the EBB was not loose or improperly tightened either. It was properly torqued down before this happened. And after it happened, I recentered the EBB on the trail and tightened it again almost to the point of snapping the tensioning bolt. But again it slid from right to left within a matter of minutes as I was riding out of the trail. It got to the point where I had to stop about every five minutes to keep loosening it, readjusting it and then retightening it again as I rode along. It sucked, HUGELY. Anyway, later I removed the EBB from the shell and because it is totally unsealed it had a lot of wetness and dirt inside. I cleaned everything out with windex so it was surgically clean. I also noticed there was a bit of black frame overspray paint on the aluminum wedge which I thought may have lessened the amount of friction it could hold. So I wire brushed off the paint as an additional precaution. Then I reassembled everything, set chain tension and tightened it back up. I have ridden it about 8 times since then and it hasn't slipped since. Apparently there is a level of water/mud contamination the EBB cannot handle without slipping. Combine this fact with the fact that it is totally unsealed and the result is that the EBB must be cleaned out periodically or it'll probably slip again. And to clean it out you have to remove one of the crankarms, which if done excessively can wear the splines or square taper interface on the crank.
Another downside is that the EBB creaks and it creaks a good amount - pretty annoying. It seems as if greasing is what people are recommending to stop this. But in my view, if it slipped with mud and water in there, I would think there's a good possibility it might slip with grease in it as has been suggested. Also, the fisher EBB's tensioning allen bolt head can get partially or wholly plugged up with mud (because it's down there in the mud with the rest of the unsealed EBB). Combine this fact with the fact that the bolt is stowed away behind a chainring bolt, you have very limited access to it, making it hard to clean out before inserting your allen wrench. And if you can't get full engagement on your allen wrench, you're much more likely to strip the bolt head, making it impossible to loosen or tighten the EBB again - which can be a real problem miles away out on a trail somewhere.
This is my first EBB experience, previously I have used track ends (horizontal dropouts) and various converted chain tensioners. The EBB has a few advantages, but with this buzzsaw/creak/contamination experience, my next frame will be a return to good ole' track ends. Overall I'm not impressed and when viewed in the pros/cons balance that all tensioning systems have, this one certainly ain't on top.