There are some of use who would say that our gearboxes are a bucket of bolts, but this is probably down to the set-up of the shifter and oil type used from the factory. Those that have changed the oil and added a short shifter mod seem to find it shifts much nicer. Me I've got used to it now (I still miss a gear occassionally if I'm being lazy)
Reliability, Only from what I've read but have not heard of gearboxes going boom! They are manufactured for Mazda by gearbox specialists Aisin who make some very good gear and supply some major European brands (VW Audi etc) The design is a compact three shaft so it means that it's quite a compact lightweight box for a 6 speeder. Some of the more technical ones here may correct me or add some more info, but I think that's it in a nutshell - Good Reliable Gearbox, not so good shift quality (which you can get used to) and something that hasn't been known to fail!