My best friend and I went to see Valkyrie, mostly because we love seeing movies that are unintentionally bad. It wasn't awful. The subject matter itself is pretty fascinating and I think they do an excellent job retelling it. The problem is there is no character development and if you know anything about that era, some of the main character's actions are a bit inexplicable. I think they would have done better to give the viewer a bit more background (apparently, he was memeber of one of the oldest aristocratic families in Germany and a career military officer; he spent time in Poland in the first years of the war, which is why he gradually turned against Hitler).
The real problem, not surprisingly, is Tom Cruise. At any minute you expect him to say "I feel the need, the need for speed" or to start dancing in his underwear. To hear him speak German, as he does in the beginning, is pretty funny. Even the minor actors were clearly better than he is.
I think with another actor, it would have been a decent movie.
Also, it seemed painfully long, even though it was only two hours.