I've not had any issues with my 10k. I just wrapped up my second burn season. I'm running two pumps and I have my demand split as close to even as possible. I've never put a clock on my burn cycles, but they seem to be pretty long. I'm sized so that even when we were at -10, I still could get 12 hours of burn time on less than 1 crammed full load. The water volume is large enough to really store plenty of BTU's for my demand.
With respect to the smoke burning twice, Yoder is right. But, I think what you were actually referring to is there is an additional smoke by-pass in the upper exchanger. The "E" series has 3 passes and the "C" series has 4. In the "E" series, the smoke is forced to the front, then to the back, then to the front and out. In the "C" series, the last pass before the chimney is divided in half length-wise, and the smoke goes to the back again before exiting the chimney.
The 10k is a HUGE stove. I've seen the 5k before, and it's really on a different scale as to how big the 10k is. I've never actually seen the "C" series yet, but the improvements I've read about are just what the "E" series needed. The oblong firebox is a great idea, as far as function goes. If they got that shape without additional welds, then it's perfect. I always seem to have a few pieces of unburnt wodd along the edges. The oblong shape would allow those pieces to be burnt more easily.