The ending to ME3 does kill the series for me because it means all of the choices I've made over the entire series meant nothing. I don't think I'll ever go back and replay ME1 or ME2 because I already know everything I'm doing is pointless.
Enjoyable, but pointless..
I've never really understood that argument, but I guess when I played the ME games I considered Shepherd to be less me than a character in a set story that I controlled the personality of. I controlled the way he talked to people or handled situations, but I knew the situations would always be the same, and I was a part of a story that was unfolding toward it's inevitable conclusion regardless of how I handled the specifics. So to me the ending was just a little bit dumb, but not a dealbreaker or something that hurt what came before it. To each his own, though.