How good do upfire subwoofer boxes sound?

It really depends on the vehicle, the sub etc. In open cabins (such as an suv) I have found that down-firing sounds the best (deepest). Between rear-firing and up-firing, I would say that I prefer up-firing, but like I said, it really dpends on the vehicle and sub. Since you are building the box, just put the terminals on the side of the enclosure...that way it will be easy to try it front-firing, rear-firing and up-firing and you can decide what you like best.