I had a Hurth HBW100 in mine originally. It slipped in forward then lost forward after it blew a hole in the bottom of the gear casing. The reconditioned ZF10M replacement lasted nearly an hour before it locked itself solid. That was fun - more fun was trying to get it repaired and eventually getting my money back from the supplier.
I finally ended up with a brand new ZF10M from Lancing Marine and while it hasn't gone wrong like the last two, it is a piece of engineering that doesn't inspire confidence. Currently it makes a lovely crunching noise as I engage forward once warmed up. I spoke to an engineer about the 'box and he reckons in my application (35hp Kubota) that with the oil changes at the specified intervals it'll last about five years, stretching to about ten if I change the lube more often and don't stress it too much.
I consider it as a disposable item, hewn from the finest block of cheese.