I've run Mary bars for most of 2005, first on a Gunnar SS and currently on an ibis Mojo.
I find them to have comfort, multiple hand positions and less strain on my wrists on longer rides.
On singletrack they have a nice point and shoot position for me. Work with the positioning to see what works best for you. Try them level and with the grips angles down.
Mounted normally they have some rise to them. Mounted upside down works fine if you need to drop the height.
That said, I still use flat and riser bars on my other bikes. i adapt to what i ride pretty easily.