Looking at it from purely an academic, story-line, tensions, rising action, climax, etc. etc. etc. view, it was absolutely horrendous. The action throughout the game ramps up to the ending, the storylines are all being wrapped up and tied into each other to come together in what should have been the climax to end all climaxes. You, as Shepard, have just fought your way through an army of creepy crawley robo horrors, and are making the final charge that will bring you up to the Citadel, the place that started the entire series, where you will fight your way through hordes of Reapers to activate the Crucible and end the war. But wait! Harbinger swoops down on the remains of your troops, cutting a path through them, leaving them mangled and destroyed before they can reach the beam. You, Shepard, Hero of the Galaxy, manage to survive, along with your comrades. No backup, no "oops, let's try again," no second chances, you have to finish this fight yourself. So you stand up, get ready to finish your charge, and...
You spend the next ten minutes firing two shots and shuffling forward in a straight line, after which you spend fifteen minutes listening to people talk at you and moving your controller sticks up or down a couple of times, then you can pick one of three directions and shuffle forwards again, to be rewarded by a fifteen minute long cutscene.
Seriously, whether the ending was contrived, didn't make sense, whatever, it could be forgiven. But whoever decided it was a good idea to slow the game down to a crawl during what should have been the climax of the whole series is a fucking idiot and should never be allowed to write anything ever again.
If they had kept the storyline EXACTLY THE SAME but made your trip through the citadel full of the hardest combat you've ever faced, around corners, through changing hallways, fighting it out with your comrades in arms, it would have been acceptable. But they didn't, and it was bad.