Watchmen was such a faithful adaptation of the comic that it was unwatchable as a film
One thing that Watchmen taught me about adaptations is you can never please everyone. You will either get castrated for getting too far off path from the original or you will get burned alive because you had no originality.
I never go into a movie comparing it to the original, you will never win because the entire time you will be critiquing and not enjoying. I have know doubt that a good number of people will be very vocal about how much they thought the Mass Effect movie sucked (when it comes out), but no one will be talking about it as a movie that quite possibly could have been a decent movie. they will too focused on the fact that Shepherd hooked up with Ashley rather than Liara, or that Joker wasn't played by Seth Green, or that Cerberus wasn't mentioned.
If we have any chance of enjoying this movie, we have to go in with the understanding that we didn't write this story line. It might even help to imagine that you are watching your friend play Mass Effect, take yourself out of the equation and it might just be a good movie.