So... wow, just finished the game a bit earlier today, managed to stay away from the worst of the spoilers (saw that people were unhappy, of course, but wanted to judge for myself), but now that I have, feel compelled to drop in my 2 cents...
I have to say, I'm pretty blown away. The last few minutes of the game fails on levels that I wouldn't have thought possible considering how great most of the rest of ME3 was executed. I don't even remember the last time I saw an ending that was as lazy/contrived/unsatisfying/plain awful. It's really taken the wind out of my sails and, rather amazingly, retroactively makes me feels worse about the entire ME series (that's... an interesting feeling).
I'm still giving a think about why it feels so wrong, but I think a lot of it deals w/ how all the "choices" given to you by the (completely deus ex machina) Catalyst just makes all the choices/challenges you've faced pretty pointless. Also weird how you're forced to pick one of 3 equally unsatisfying, equally out of character options that really takes the illusion of agency out of ME. There's also no real range of difference whether you did the right things, the wrong things, or skipped most of it.
In terms of execution, considering how well the in-game character deaths (Mordin's, etc) were handled, and how character-driven the core of ME is, I was surprised by how completely how the actual "ending" doesn't seem to reflect that at all. I mean at the end of the day, I care more about my team than the "fate of the universe," especially w/ the utterly cliched choices. To me, all the endings were basically the same except for different color lights. And who thought the DX-style choose-your-own-adventure ending mechanic was anywhere remotely a good idea? Personally, I'd have been much more satisfied if an ending ended up being formed from the chain of choices you'd made throughout the game(s).
Anyway, while I'm mostly disappointed with the lack of emotional resonance the endings provide rather than the specifics, I do have to chime in with some those. The Catalyst reveal is just a horrible deus ex machina - the first thing that popped into mind was the Architect from the Matrix trilogy. That's not a good thing. I'd much rather have talked to Harbinger at the end. At least I'd get some closure w/ the Reapers... as it is, well, they're inert no matter what happens. Also, so many things that don't make sense. Where is the Normandy even going to, aren't they fighting a battle on Earth? If you brokered a truce between the Geth and Quarians, why can't you argue with the Catalyst? Why does any of it matter if all the options blow up the Mass Relays? What are the consequences? What happens to the fleets? Hey, wouldn't the saving the Rachni/allowing ansible connections come into play? What, my Liara LI just got incinerated with no notice on Earth? What, oh it's OK she's on LOST planet with everyone else. Too bad she's not sad about my Shepard dying. What sort of magic is synthesis in the ME universe again? Glad no one minds that I decided to spontaneously recreate all life in the galaxy.
There's probably more, and I could probably torture myself reading all the threads in the forums, but, well, there doesn't seem like there's much point. Signing off from a (former) ME fan that's just pretty bummed right now.