I have the BME50. Ran it for 2yrs almost. No prob whatsoever. Broke it in per instructs. It now runs a 22x10 ProZinger(I think thats the brand on it now) right at 7000r's. Pulls my 15lb Midwest Extra out of hover(what I can do of one anyway, I get a good 15-20sec sometimes) with authority. Starts every time. You can't go wrong with them, they have to be as good as the best out there. I don't see how it can be better. I have not even come close to the threshold of this engine either as far as prop selection goes. Ive heard I can get alot more out of it. Just not into taking stock out in carbonfiber props right now. Not that I break any either...matter of fact, still have the original on the plane. It works so well, and the engine/prop pull it just fine...why switch.
You'll like it. I wouldnt hesitate to buy another if I needed a 50cc class engine. I'm looking at 80-100cc for another project though at this point. Take a look in the market-place here too, you might pick up a used one for decent price. Dont know. They arent cheap though at about $625+ , that is price with the muff and shipping I think. The engine goes for $499 and you can get the "banjo" style muff for about $49, or the pitts(the muff of choice for GiantScale Aerobats) that one runs $99 I think. Good luck with...I hope its as good as mine....I really like mine.