I think it depends on what your definition of a happy ending is. I would have been fine if Sheperd was required to die in all the endings, but at least give some indication that what I did over the 3 games meant something. Like say if you get enough war assets and work really hard, shep may still die but you can witness the different species rebuilding since the Reapers weren't as successful.
I would not have wanted a really really happy ending because it would not be realistic given the destruction the Reapers are capable of, but they should at least give some indication of the state of the galaxy. IMO destroying the relays were the worst thing they could have done. Killing characters or destroying whole planets is one thing, but totally screwing up the Mass Effect universe was a very bad decision.
One thing I absolutely hate is a
"Gainax ending."This type of ending rarely, if ever, works, and contrary to what authors think, it does not make your story some kind of profound brilliant high art that people will be writing papers on 100 years from now. This is also true of nihilistic sad endings which have become a cliche in the past few years. Both of these endings make the story feel meaningless and confuses the audience especially if the story is straight up space opera like Mass Effect. It does not fit with the rest of the series.
A lot of Gainax endings have something like all the people in the universe merging and becoming one being and living forever in eternal peace. This is pretty much what synthesis is. I guess I just feel that this is a cop out, and usually an indication that the author had no idea how to end the story.
Conflict, stress, and hardship build character. Even when it is clear there will be no sequels, I enjoy imagining how the characters dealt with the situation and what came after. This is what people are interested in when they get into a series like ME. The characters, the relationships between them and the universe they inhabit are the important elements here. Merging all beings into one destroys what makes the universe interesting, and absolute nihilism makes the audience wonder why they even paid attention to the story at all. It's just not good storytelling.
So I guess I got slightly off topic, but for the tl;dr crowd :
I'm not ashamed to say I want a happy ending. It should have been an option, though showing sacrifice would have been important.
I think there should have been a variety of endings based on what you did.
I hate the ending mostly because it is another example of either a failed Gainax ending, or is just straight up nihilistic without needing to be so.