I loved the ending and though it was genius.
For me, the first time I realised I loved Mass Effect and something set it apart from other games was in the first game when you choose the fate of the Rachni Queen. Play it safe and make a whole race extinct, or do the 'right thing' but potentially put the rest of the universe at risk further down the line. Such a tough decision which was so morally grey with potentially awful ramifications is what made me fall in love with the series. So fast forward to the end of the trilogy and the last thing you have to do is make an essentially impossible decision, it blew me away and really messed with me in the best kind of way.
As I said, lack of epilogue was annoying, but they corrected that anyway. As for decisions in the series not mattering? They absolutely did. There is no way every little thing could have influenced one ending scene, it's way too complex. The choices mattered and the results of which were seen throughout the third game, where everything was resolved on the way to that final scene.
I understand why people didn't like it, but I don't think it deserved how much negativity it got.