My Ofna box with the dual 550's and a 12v hobby battery did have some trouble with the break-in on my engine when it was brand new, but that was only for like two starts (I will add that I couldn't turn over the engine with a full grip on the flywheel when it was new either.) Other then that it's been a champ. I did end up replacing the power switch with a better one from Radio shack though, but I have really nothing bad to say about it. It's a workhorse.
The single big torque boxes are nice too though and usually have no issues. The dual 550's boxes usually get up to speed quicker then the single motor boxes though as far as I've observed.