I just got back from the movie, and it hasn't really sunk in yet, but I have gut reactions:
Character death #1, I'm OK with. I was almost expecting it just from trailers, and certainly it was foreshadowed in the movie. Reasonable for the plot, makes sense.
Character death #2, I am very much not OK with. No foreshadowing. No preparation. Not even any purpose in the grand scheme of things. Just a completely needless loss of a wonderful person.
I recognize that it shows exactly what was intended: that it was random and needless, and that bad things happen to good people, and sometimes there's no time to say goodbye, and that the crew had to sacrifice to get what they believed in. I also recognize that the timing created the necessary doubt for the rest of the action, since clearly no holds were barred and it could happen to anyone.
I'm still not OK with it.
I think that, even more than the plot points, are what makes it seem very clear to me right now that This Is It. No sequels. No picking up the show for another season later on. Serenity is still flying, but we aren't going to be invited along.
Even aside from that, this was still darker than I had expected. jaedeth had asked me to let her borrow my DVDs of the show, and I was reluctant to lend them out, although I'm going to. Now, I think I'm glad. I think it's going to be a while before I can watch them again anyway.
In a few days, I may do a real review, when I've had time to absorb it and reflect upon it.