In my experience they work well for a standard set up, i have them on my 2008 Terrago disc and they pull up very well and down some serious hills with no noticable fade, but they have a common fault, the fluid reservoir which has the round knurled (??) adjuster can sieze up and break it seems common when the bike hasnt been used for a while.
This happened to my rears, my LBS said they have had quite a few that have done this. I had to replace my rear with an Avid Elixer 3.