It was pretty shit. Seriously, had they not just made the starchild into some kind of AI that pops up and goes "Please don't, reapers reap because of reapers reasons!" and then you either fire it or not. If you fire it, machines wiped out have a sob etc., if you don't, you all join the reapers in their reaping fun.
In fact, I'd have preferred it to simply damaged the reapers, as that would save the tone of the end of the game, as Shep could still go out with all of his allies to fight the damaged reapers in a final, desperate attempt. Mass Effect was almost Lord of the Rings in space, all of the fractured races of the galaxy's being banded together in the end to try and survive against an encroaching evil that will kill them all.
But instead, philsophy, and either you wipe out everything (best option), you pull a fucking wizard out of your arse and make magic organ-sythenic-mongoloids (seriously, how the fuck does it even happen?! Everything else, even the literal biotic magic is explained, but nope, synthesis is literally just magic) and Shep leading the reapers (eh, kinda weird, pretty much destined to end terribly when Shep starts to go a bit nuts.
Also, fuck Bioware SOOOOOO much for including the Stargazer ending. Seriously, fuck that giant pile of shit. They included it because of all of the people who said they wanted a choice to simply say "No" to Starchild and fight the Reapers. The moment you do so, it fades to black and then reveals you all died and the next generation is telling stories about you. Seriously. Fuck you Bioware, it's just a giant middle finger to everyone who formed an army and actually wanted to see it fight against the big bad. Hell, Mass Effect 2 did it better with the ship upgrades, where if you didn't get enough your ship would start to fall apart and people would die, but if you worked hard enough to get all of them you scrape through with everyone mostly intact.
Urgh. Lastly, if anyone knows, why in the bleeding fuck does Shepard need to gather points to get the various endings? Seriously, if there is a reason I can't remember. At first gathering the points made sense as they represented the war effort against the Reapers, so having a low score basically meant losing the war. But if at the end you just choose what to do regardless of what you did, why does Shepard need his magically points exactly?
EDIT:
However, the Citadel is literally the greatest DLC ever for anything. Seriously, that DLC is fucking brilliant. It lets you enjoy and really get into the characters of everyone you have met and bonded with throughout the story, and then you have a bloody stupid but fun plot where you fight your own twin, and most importantly the Citadel battles involve your ENTIRE team fighting. You get to see all of the people you've found and trained work together to kick some evil clone ass. Seriously, good DLC. Still, shit ending. Kinda made worse by the fact I loved that damn DLC so much.