I’ve had a set on my Fatty for over 2 years. They’re fantastic when they work; good power and great modulation.
They have needed quite a bit of maintenance, though. Probably have bled them about 8x so far, including a professional bleed by a shop who likes Avid brakes. They’d just get spongy after a while no matter what.
The best thing that I found so far is this http://www.pinkbike.com/news/tech-spotlight-5-minute-bubble-bleed-2014.html
In my experience, the bubble bleed is all that it takes to get a little bit of trapped air out of the master cylinder and make the brakes work great again! Quick and easy to do; much simpler than a full bleed.
The other thing that I have noticed is that the pads have to be replaced when there is still a bit of material left as the lever goes spongy. I don’t think that the master cylinder volume is quite big enough to account for fully wearing out a set of pads. I just replaced my rear ones last week with a spongy lever; they worked fine again once the pistons were reset with new pads at full thickness.
And while Avid pads aren’t super expensive, I’ve been real happy with Clarks brake pads that I got a lot cheaper from On-One.