Mass Effect is just severely overrated. SEVERELY. 

the brand dude, Final Fantasy brand on PS3 is much stronger then Mass Effect brand on any platform, its not quality that sells, people ought to know this by now.

So without reading what anyone else has written I will just say why I and some friends of mine didn't play/buy it...

As a group of 5 people including me, we were huge fans of ME1, everyone bought the game after playing it one bit at one of our places... then ME2 release... one of us goes and buys it day 1, puts in the game to play it, plays it for 5 hours and calls us others with the following words (free translation): "Don't bother buying it"... Some days later I went over to his place to play it myself to get to know what was wrong with the game and to play it myself and after the first few hours I also knew what he meant... It's gone completely shooter... eventually only one of us played through it after getting the game from our first adopter as a present and even he said it isn't that great... I have to say I am still thinking of playing through it once it gets cheaper to buy on PS3...

EDIT:

Oh! And while I think FFXIII is a bad FF it is still a good game/RPG, ME2 on the other hand is a bad ME/RPG and a good shooter with story

Mass Effect 2 isn't an RPG, it is an action adventure/ third person shooter.

oh crap
unbelievable...

Sales of ME3 vs FFXIII-2 will explain, and correct, any misconception that FFXIII is somehow an objectively better game than ME2.

TBH I think if the ME franchise had been PS360 multiplat from the beginning there's no way FFXIII would have outsold ME2. The fact that ME2 has crept its way to 1 million o PS3 is testament to it's quality.

Nostalgia. 

People intentionally ignore the fact that FFXIII-2 is a spin-off, FF spin-offs alyways sell less ( look at X-2 sales compared to X ), let alone being a spin-off to the probably most hated FF

Now that can't be true, even if the PS3 version sold equally to the 360 version, the total would be 5.34, in comparison to 6.58 for XIII.

FF13-2 is a sequel not a spin off.

its irrelevant, since ME2s sales are already above those of FF13-2. and ME3 is going to launch similtaneously for the PS3 and the 360. no chance FF13-2, FF13-3 will outsell those games. maybe Versus wont either.

it was also unfair for the OP to compare a relatively new RPG to a well established JRPG. could the OP explain why Skyrim has sold 10 mil copies compared to FF13s (however many copies). Mass Effect 2 (and soon 3) have sold more than FF13-2.

Yes sequel is the right word , however it's still not a main-line FF

I'm pretty sure XIII-2 is ahead of ME2 in the same time period, I can't bother enough to get the data though.