It's far from the worst movie I've seen, but it was probably the worst one I watched at the cinema in 2005. It held together reasonably well and the effects had nothing to complain about, it just didn't really have any point and the Rock was unable to rescue it from mediocrity.
The scene with 'I've got... one round' was pretty cool, if for no other reason than the BFG being just as useless in the hands of a newb as it is in the game, and for the same reason.