Just following on from what Hey Joe said..
Yes in general there are a lot of game adaptation movies out there that do suck worse than a £2 prostitute , but after all, there's a lot of normal movies that suck worse (Doomsday 2012 anyone?).
I think that Mass Effect has enough depth in it's storyline to pull off a reasonable movie, and as Amnestic mentioned, you wouldn't be able to fit all of the charactors backgrounds into the story as the game has hours upon hours of gametime, especially if you did all of the planet-hopping sidequests.
Now I liked Mass Effect and if there was to be a movie insipered from the game there are some things I would suggest to the creators as to what would be good to include.
These are...
Have someone GOOD play as Shepard, maybe Vin Diesel? Or at least someone who can act the tough guy and pull it off.
Make the Rover look a bit more butch instead of the usuall 8 wheeled box with a gun on top, and have it fight at least 1 of those worm things that spit acid (I forget it's real name).
Don't cast all 6 charactors, just cut it down to about 2 or 3 co-stars, preferably Wrex, Garrus and Ashley.
Find a real female actor to play as Ashley, I'm sick of the big-titted, blonde skinny bimbos that you usually see in films, find someone like Jessica Biel(drooool), Natalie Portman or Halle Berry. A distinctly femanine charator who can keep up with the macho-ness of the guys.
Have someone like Angelina Jolie to play as Benezia, sexy yet sinister.
DON'T change the Normandy.....at all!! It's a good looking ship, it doesn't need bastardised into anything else.
Give them armour from the game, for example, the Spectre armour looks cool.
Try not to deform Saren Arterius, he's quite an intimidating "bad-guy" which is a nice change to see instead of all the limp-wristed push-over evil-dooers. Take Iron Man for example.
And as far as the sex scene goes, it's down to personal preferences, I don't think sex has anything to do with decent action films, but then at the same time, I'm a bloke and I'd like to see Jessica Beil get jiggy!! Hey, I'm being honest ok?
I could go on but I need more coffee and smokes :0) but you get the idea.
We have the technical equipment to make the special effects look shit-hot (Transformers) but they need to spend the money, so unfortunatly it looks like it has to be done by George "fuckbag" Lucas (-_-)