The ending is almost exactly like An Occurrence at Owl Creek Bridge except we're led to believe the alternate reality that Jake Gyllenhaal's character finds himself in at the end is real. It's basically the Hollywoodi-zed happy-ending version of An Occurrence at Owl Creek Bridge. If it's not, then it's all a delusion in his mind as he's dying, à la Mulholland Dr. But I think we're given enough scenes from Vera Farmiga's character's POV in the alternate reality that we can discount it as a delusion.

First time ever I think I wanted a film to end at a very specific point. People say the same about Minority Report and AI, but hot damn if this film ended on that freeze frame it would probably be in my top 10 of the year. It was depressing and yet truthful which is honestly all I ask of a film. I don't want to be told a comfortable lie. The characters had already discovered the point in living those final seconds as if they mattered, and that's all you could ask. Adding some stupid happy ending bullshit that just brings up more questions that don't make sense annoys me. Could so easily just cut the film off on that freeze frame and no one would ever know.

on the freeze frame, but I did want CPT Stevens' time in Sean Fentress's body to end there. I understand why he was able to save the people in that one universe/timeline (out of an infinite number), but keeping him in the body like it was somehow the end of Avatar rang false. The rest of the ending--with the note to Goodwin and the frustration of Dr. Rutledge--I enjoyed.

I think people aren't getting the ending...it's essential to one of the points in the movie that an open time/reality loop is created, which makes sense as this was heavily influence by Quantum Leap. Here is an explanation from someone on imdb...."Goodwin tells Colter in the original universe that everything will be okay and in the alternate universe he lives on in, he tells alternate reality Goodwin to tell alternate reality Colter "Everything will be okay" when Source Code is activated in that alternate universe (via the email). Yes, it's an open time loop. The Colter in that universe will be activated for a different event and Goodwin will shut off his life support and allow him to live on in an alternate reality to that one and it's implied that Goodwin from the original universe had been contacted by a Colter from an alternate universe before the beginning of the movie."

the extra few minutes of Colter and the girl walking around the alternate reality. I think pretty much all of this can be figured out from the point of the freeze frame shot. Colter and the girl walking around, and the confirmation that Goodwin received the text/email were just standard payoff moments used to explain everything to an audience that may be confused.

If the movie ended with the freeze frame, then the weighted implication would be that the alternate reality is not self-sustaining, and Colter is just plain dead. That's a completely different ending. Feel free to argue whether that's a better ending, but it's not the same ending. The filmmakers wanted to establish that the alternate reality was self-sustaining, so you needed scenes in the alternate reality after Colter's body died in the original reality.

That ending would be completely ambiguous. The viewer would be free to think what you've posed or what roastbeef posed, given what the movie had presented. So yes, the ending would be different (I never said otherwise), but my point is those scenes weren't needed in order for the viewer to know what happend, should they chose to believe the alternate reality was self-sustaining. It might require some thought to puzzle it together, and lord knows American cinema can't have that.