I haven't had any problem with them. A couple of important points though. First, I'll try them on a low res figure (for rendering speed). When I import them there's adialog box that asks if they should be aligned with the X axis or the z axis. I'll apply it to the figure one way and render it to see if that works. If not I delete all the animation and re-import it using the other axis. I've never not had one of those options work. Then I add the text "X Axis" or "Z Axis" to the folder they're stored in so in the future I can get it right the first time. Another thing you have to watch out for is that they don't always interact with the floor properly. If so I'll select sections of the motion graph and move them up or down as required (reshaping it).

Before they made me they broke the mold!
http://home.roadrunner.com/~kflach/