Well I finally saw the film in its entirety today and I will agree with most others that it is a pretty solid film. I would rate it a B+, or give it ***, as well as my friend who saw it with me. We were both impressed, especially since in his close circles he had heard negative sentiments regarding the film, and I had read a mixed reception from most critics.
I would have to agree with JKTim over the second act. I thought it was slightly sluggish and not nearly as entertaining or engaging as the first act (and the rousing opening set piece set in Africa, which I heard was a last minute addition that Singer shot when they did re-shoots, and I'm glad he did, because it's exactly the type of opening this film needed) or the third act, which is the best part of the film. I think if anything the writing could have been tighter or some scenes where we see Stauffenberg creating Operation Valkyrie and assembling the "team" could have been cut, since it slows the film down. My friend did not share my sentiments, which I don't know is a good or bad thing, or whether or not I'm being nitpicky here.
However, everything else I was very impressed with. Bryan Singer is a stylish director, one who knows how to work with actors, set up a story and focus on the nuances that make characters feel believable and real. He's also talented at crafting a superb thriller. I feel like he hasn't done a real thriller since The Usual Suspects and while Valkyrie isn't as clever a film, it has more depth and substance by making the characters feel real, with flaws and traits, which only adds to the emotionalism of the climax. By this film's end, you're fully invested in the characters. Tom Cruise, Bill Nighy, Terrence Stamp, and others all do incredible work. If anything, I was surprised by the small roles of Eddie Izzard and Kenneth Branaugh, especially.
I was actually kind of suprised by John Ottman's score as well. I'll definitely be getting it. I was half-expecting a sort of lush, orchestral, "time period" score, but instead I heard a distinctively modern, edgy work. I liked it a lot, and I felt it really added to the proceedings without becoming melodramatic or overstating.
Side-topic: To answer Barbados Slim's question, the reason why it took so long to make are a couple of reasons. One, it took a long time to convince Germany to even let the production film in Berlin and utilize the real locations that were featured in the film (because of Tom Cruise and his ties to Scientology, which are looked down upon by Germany). Two, Singer did everything real. The airplanes, the locations, the set pieces, everything was done as naturalistically as possible without the usage of CGI. Third, footage got destroyed which required Singer and Co. to re-shoot numerous sequences. Singer also allowed this to reconceptualize certain elements, and re-craft the film after earlier negative advance screenings. Add all of those things up, as well as MGM constantly delaying or pushing the film back, and there you go.