Well, you said you look at movies as movies before adaptations. So what's wrong with Ebert judging it just as a movie (and a bad one at that)? It also meant he had a totally neutral opinion of Doom, so could go in without the bias of "Well, it's Doom, so that bumps it up a few points."
Where are these lemmings going? The
Super Nintendo Super Shire! Hop in line and follow them there!