I don't know about all the nay-sayers (nor do I care) - I thought Valkyrie was really good, and that includes Tom Cruise as Colonel Claus Graf Schenk von Stauffenberg. He plays well as a man sure of himself and of his convictions. The rest of the cast was great too - Tom Wilkinson, Eddie Izzard, Bill Nighy, Terence Stamp, Kenneth Branagh, Tom Hollander, Thomas Kretschmann, Christian Berkel, and Jamie Parker - and I learned a thing or two about the assassination attempt of July 20, 1944. I knew about that, and that it failed, but I didn't know that these officers had been trying for a couple of years to assassinate Hitler, and that he kept eluding them because he kept changing his plans and schedules. I also didn't know anything about the "Valkyrie" part of their conspiracy - how they were planning to overthrow the government after Hitler's death. Good stuff, well played.
Oh yeah, and Firstshowing.net has an interview with screenwriter/producer Christopher McQuarrie that is well worth a read for lots of background and historical info.