It's been said many times that what works for a comic/manga/anime won't necessarily work for a live action movie; I agree with that, and I think that the Oozaru transformation is one of those things. It's not that it couldn't work per se, visually... the K. Kong example is well put; it's that I can't see an Oozaru transformation doing absolutely anything to advance this movie's plot.
Let's get real: by the time DBZ (the series) began, there was little use for it anymore. It did nothing for the Gohan training story, little else for his second transformation in the final stages of the fight against Vegeta, and only the Oozaru Vegeta bit had any particular purpose, plot-wise (i.e., to make Yajirobe get finally involved in the fight). And I can't even remember now if any other Saiyan ever transformed into Oozaru again, after that.
In general, it's way too clumsy. Much more so when the transformed Saiyan can't retain his conscience, because then we have just a huge monkey smashing rocks like crazy, until someone else (villain or good guy, doesn't really matter who) decides that it was a bad idea to let him transform into Oozaru in the first place, and either chops his tail or blows whatever acts as the moon. But even for self-controlled Oozarus like Vegeta (which, as has been said too, has little chance of appearing in this first movie anyway), it's still clumsy and detracts too much from the suspense of who's going to prevail in the fight.
Besides, current SFX technology or not, it'd still be too expensive an effect to use for just a few minutes, which is the maximum amount of time an idea like this could be used for the film.