The 787B is one of the most stable Group C prototypes out there, as far as I'm concerned. The Toyota MINOLTA 88C-V holds the title of most jittery, if you ask me.
I love the high-strung wail of that R26B, the smooth power delivery, quick acceleration and smooth cornering (with the aforementioned softened spring rates, naturally). The Black Beauty™ 787B has less power, dirty oil and a warped chassis (for those who didn't know) yielding higher points in it's original condition. I actually like it in all the prototype events, like FLK said.
The coolest thing about this car is its fuel and tire economy. With dirty oil, it can go a full ten laps per tank of fuel and set of R3/R2 tires at Sarthe 1, and the brand new Renown version can go a full nine before needing to enter the pits. About the only prototype that can approach that is the Peugeot 905, and its massive torque curve and interminable shift lag make it so much more difficult to drive.
The Mazda 787B is a winner, in my book.