Unlike a lot of people, I wasn't too pissed off about the ending when I got there. The fact that you couldn't turn down the Catalyst's options was understandable - those were the only things that the Crucible was capable of doing, Shepard was barely conscious or dying, and presumably if you just sat there and waited, the Reapers would take notice and blow up the Crucible while leaving the Citadel intact for the next cycle. Even the lack of a final boss wasn't too bad, seeing as it was a similar problem with Halo 3 - of the two possible final boss candidates, one (the Illusive Man) wouldn't be able to put up much of a fight, and the other (Harbinger) would have been impossible to fight. Knowing that it's possible for Shepard to survive in at least one of the endings helps to soften the blow.
It was only a few things that annoyed me - Harbinger leaving, for one (it doesn't explain why Harbinger does that, and it doesn't make sense), and the whole ending cinema sequence. A lot of it didn't make sense (Joker fleeing Earth with the rest of the party apparently on board the Normandy somehow, the fact that the only difference your choices make are what colour the beam turns, the lack of any closure on the part of any of the cast or any of the forces that you had built up over the course of the game); I assume some of it will be covered by that Extended Cut DLC, though. The scene at the end, with Buzz Aldrin, seemed to indicate that galactic civilization got back on its feet, so it's not like all possibility for sequels or spin-offs have been destroyed.
Of the three endings, I have no idea what Synthesis involves, but the other two seem fitting - though they jumble with the Paragon and Renegade options a bit (the colour schemes and who the endings are represented by are completely contradictory), both seem to be rather effective ends to the trilogy. It seems to evoke Legion's loyalty mission in Mass Effect 2, as there isn't really a right answer and the choice is between brainwashing or destruction, and while the Control ending sees Shepard sacrificing himself for the needs of many, the Destroy ending potentially sees Shepard sacrificing many for the needs of a relative few (depending on whether the Catalyst's prediction of a robot uprising that results in the extinction of all life in the universe, technologically advanced or not, comes to pass).