This movie will bomb -- as in flop.
Let's be honest; the Mass Effect story arc is a tired trope in sci-fi. I've only played the second game, but reviews tell me that it was the best of the three. The Only problem is that even ME2 did not have very strong characters. That's right, there were no strong characters.
War of the Worlds and Fantastic Four: Rise of the Silver Surfer had the same plot, and were heavily promoted; but they were not very good, and the latter had a disappointing reception. WOTW tried humanizing the experience, but that only made the plot crawl through 2+ hours. Fantastic Four tried to be everything Summer Block Buster, but ended up being a waste of my time.
If the writers are smart, they'll abandon the epic, meandering story and let the imagination of sci-fi work its effect on the viewer, like in Star Trek. The only problem is that ME does not have a generation of imagination and an assortment of characters to work with.
Otherwise -- and this is what I'm convinced will happen -- we're going to have a hybrid Bourne/Transformers series: static, one-dimensional characters fighting government-alien conspiracies while trading tired one-liners and running through battle fields in slow motion.