The movie is like others such as Apollo 13 where the broad strokes of the events are reasonably well-known, while the details are generally not. Some of the tension is sucked out of Valkyrie since we know that Hitler was not successfully assassinated. Thus, we know our protagonists are going to fail in their mission from the get-go. What we're interested in is how they fail, and how agonizingly close they get. That's where the tension comes from, and it's done well.
Several times in the film I was reminded of Alfred Hitchcock's "Bomb Theory":
There is a distinct difference between "suspense" and "surprise," and yet many pictures continually confuse the two. I'll explain what I mean.
We are now having a very innocent little chat. Let's suppose that there is a bomb underneath this table between us. Nothing happens, and then all of a sudden, "Boom!" There is an explosion. The public is surprised, but prior to this surprise, it has seen an absolutely ordinary scene, of no special consequence. Now, let us take a suspense situation. The bomb is underneath the table and the public knows it, probably because they have seen the anarchist place it there. The public is aware the bomb is going to explode at one o'clock and there is a clock in the decor. The public can see that it is a quarter to one. In these conditions, the same innocuous conversation becomes fascinating because the public is participating in the scene. The audience is longing to warn the characters on the screen: "You shouldn't be talking about such trivial matters. There is a bomb beneath you and it is about to explode!"
In the first case we have given the public fifteen seconds of surprise at the moment of the explosion. In the second we have provided them with fifteen minutes of suspense. The conclusion is that whenever possible the public must be informed. Except when the surprise is a twist, that is, when the unexpected ending is, in itself, the highlight of the story.
Valkyrie knows how to generate suspense, and it does it well. It also seems to have done a reasonable job remaining faithful to historical events (at least about as close as a Hollywood blockbuster is going to).
It's a depressing movie, though. You'd think it might be uplifting to think of the plotters as heroes, motivated to kill the monster Hitler by lofty notions. But they each had their own reasons, and they were guaranteed positions in the new power structure if they pulled it off, so it's likely that they were motivated by a mixture of base and noble reasons.
Still, I never found the movie boring. All in all it was a solid experience, and I'd recommend it.