Martin,
Careful, the Japanese maples are quite expensive as compared to 'any' old maple. For some of the bigger specimens, they make fig trees look cheap :)
Only thing I've bumped with my mower (knock on wood) was a bit of a rain litre. Oh, and I've nailed a few dog toys (mostly spongy balls, or tennis balls w/ rope on them). Dog kind of leaves them around, and when the grass gets high, cant see them sometimes.
Always fun to hear a thump and watch a ball shoot out the side of the mower :)