I know it's a bit late to join in this conversation, but I'm going to anyway!
I saw Source Code today, and thought it was a fab film. The only thing that spoiled it for me was the ending, as many people have said already. There are many reasons for this, and I'm not clever enough to go into all the science-y stuff, but the main reason is that we were treated to a fast paced action packed 80+ minutes, and then at the end, for the last 10 minutes or so, it all got very slow, and I didn't like the contrast!
For me, the perfect ending would've been