I'm not a super fan/fanatic of the Mass Effect series, so I believe I can look at this issue a little more objectively.
I think I'll start with the cons first to appease the majority of ending haters. The ending, as in the last 10-20minutes of ME3, sucks. I agree with that. Plot holes popped up everywhere. Random characters learned to teleport. Omniscient god child... I have to say that the ending was unimaginative and very thrown together. It is pretty much the definition of a "deus ex machina" to wrap up a series they did not know how to end. However, that is only if it is taken at face value.
I now bring up the Indoctrination Theory of the ending. Go search YouTube if you don't know this Theory. The Indoctrination Theory is genius if it holds true. I really applaud the people who scoured the game for evidence to back it up. I subscribe to this theory too, since I thought the little kid was a hallucination from the start since playing the ME3 demo. However, here is where Bioware/EA fails: they do not confirm the Theory. It's so f-ing simple to get people off their backs; just a few words: "yeah, that's correct" could completely revitalize their image. I don't know if they are trying to hype up some future DLC with an actual ending, but their actions now are just stupid. This is the kicker with the Indoctrination Theory. If it holds true, then the game HAS NOT ENDED! That little clip of Sheppard gasping in the rubble at the end of the Renegade ending alludes itself that Sheppard better get up and finish the fight. An
Okay, now for the pros for the ending...there's not that many, but I'll try to be fair. I'll assume Indoctrination Theory is false here. The ending is a "deus ex machina", yes, and that is fine. A "deus ex machina" is a legitimate plot device in plays and literature for ending something overly complex. It might be a cop out, but what other way can they end this? The Mass Effect story is immense and convoluted. There are so many stories entwined together with a pretty weak thread tying it all together. That thread is Sheppard/Normandy and whatever he focuses on will be the focus of the entire galaxy. ME1 was Saren. ME2 was Collectors. ME3 was Reapers. Yes, they are all actually the same thing. Anyway, it all focuses on Sheppard and the ending must focus on Sheppard's actions. It really does not matter what allies you've gained or what enemies you've made. Everything still depends on whatever Sheppard decides to do, and as always, there are still only 3 choices: paragon, renegade and neutral. That is how the game has always progressed and that is how it has ended.
Lastly, my last point is that I think ME3's story as a whole is the true ending not the last 20minutes of it. Playing through the ME3 story and seeing your actions in the previous 2 games play out is the true ending. This is really not as evident to individual players or people who only played the 3rd installment. Did you know? You can save Mordin. You can ally with BOTH the Quarians AND the Geth. You can save the Rakni AND prevent them from rebelling later on. Who joins your cause and who hates your guts are all repercussions of previous games. I played ME2 on PC, so I kept thinking how things should have gone differently in ME3 if I could have imported. That last 20minutes was a cop out, but everything else leading up to it was a beautiful piece of intricate story telling.
I'll sneak this into the end and hope no one actually read this far.
Disclaimer: I have never played ME1. I did not finish ME2 (played up to the suicide mission then dropped it). I have not finished ME3 yet since I am grinding the multiplayer. I just spoiled myself most the endings through YouTube videos while waiting to grab this game at half price.