I liked the ending, but I know why people don't like it. It gives you no closure and leaves everything intentionally ambiguous. I like that in stories, see Children of Men for the first example off the top of my head. My Shepard set out to stop the Reapers, and he did that. He may of set technology back and destroyed galactic civilization, but in sort of a Nihilistic way, maybe that's okay. It's like the one ending in the original Deus Ex where you stop the New World Order but also blow up the internet and all forms of communication in the process.
The game really had an effect on me. Like I said, I play renegade Shepard who's a ••••• and shoots people, but he basically wants to save the day and doesn't do the most horrendous stuff (for example, I refused to kill that Turian general to get the Blue Suns on my team. BTW, what happened to Zaeed? He was cool!).
But, over the course of the game, I had to betray the Krogran, shot Mordin in the back, then had to kill Wrex. Then betrayed Legion and the geth. It really put me in a blue mood and I had to chill out and quit the game for a minute and think about some stuff.
My Shepard was cutting corners to save the galaxy, but at what point does the end stop justifying the means? In ME1 and ME2, I had fun kicking guys out of buildings or torturing that guy at the police station or killing the rachni queen (just a bug, who cares), but in ME3, the renegade choices got harder and harder.
The death of Mordin hit me hard, and the death of Wrex was hard too. When Wrex said, "I should of known you were lying to me on Virmire, Shepard," that hit me like a sledgehammer to the stomach. I felt so ashamed of myself.
Stuff I didn't like - Miranda's involvement in the plot felt added at the last minute. Miranda was cool and I should of been upset at her death, but I wasn't. It felt totally forced in. Plus, I think she had all 5 minutes of screentime.
As for changing the ending, I don't think they'll do it. Bethesda changed the ending in Fallout 3 to necessitate the inclusion of DLC. I think BioWare will stick to their guns with this ending, and I think they'll just add DLC by having you do it during the final mission.
Also, my save game didn't import right. I didn't figure this out until about 10 hours in, but Grunt and Thane were dead when they survived ME2 in my game (their names were on the wall in the Normandy, couldn't figure out why) and my romance with Ashley didn't carry over for some reason. It was weird because I had something special with Ash in ME1, and had that tense in encounter in ME2, and now she's acting like we're just friends in ME3 with no discussion of what was between us. Just like my real life, AH, too much information! Anyway, that ruined the romance stuff for me because Ashley was in the "friend zone" for the whole game when I expected us to be bunk buddies.