Cam, having the non-bull moose version on my clem, I tend to think it is mostly a positive. I ended up with and prefer the ends rotated downward much more than the fixed position on the b.m., primarily because it makes the 3rd grip position (at the downward curve) usable and comfortable. If the main grip section is set more horizontal, you have to contort your wrists oddly to grip the curve, and your hands will want to slide forward. I also found i needed a longer stem extension than the b.m. would have provided. I got the 130 which is the longest riv sells, but would (and may) prefer 140 or 150 if i can find one. Likewise, i much prefer the negative rise of a normal stem. The 120mm /90 degree effective stem on the b.m. would be too high and way too close for the amount of rise and reach-back those bars have.
On the other hand, i do agree that the loss in rigidity is a negative. I find my riding technique restrained by the flex of the bar (technically, i think its more of a "torsion" than a "flex"), and am reluctant to perform out-of-the-saddle sprints unless I'm gripping the curve part mentioned above. Other than the regular bull moose, it's the only adequately-wide bar riv sells, in my opinion, and i think the lack of stiffness is the tradeoff. Great bar overall.