Does anybody else work on the engine first? Wouldn't that mean that if you did the engine first you would have to hack your own code to get it into a playable state?
I do the engine first... If nothing else, the major show-stoppers will be in that area, so if you really run into something that you can't figure out and makes you discontinue the whole thing, at least you didn't also waste a whole bunch of time on the 'easy' things that you could have done later on in the process.