If you don't have plans to see this movie, you can check the spoilers here and then come back.
I think it's safe to say Robert Downey Jr. is currently in a zone where he could make even a bad movie at least watchable on his quirky charm and charisma alone; here, I think he takes a so-so effort with some definite strengths and elevates it to the point where you at least stay entertained consistently despite some overall shortcomings. Certainly the Sherlock Holmes on display here is an excellent use of Downey and a character he can really sink his teeth into, as he uses facial expressions, physical comedy, vocal tics, varied fighting styles and more to make the great detective a unique and enthralling hero. The sequences in which director Guy Ritchie and Downey break down Holmes' methodology both for fighting and deduction are quite fun and I kinda wish there was more, but I get that would reduce their impact. Where the big problem for me came from was that the plot and pace of the story meandered somewhat for about the first three quarters laying down clues and establishing Holmes, and then ratcheted up to the kind of intensity I was expecting from the trailer only for a satisfying final act; I get that the general idea of a detective story necesitates this sort of form, but I guess I was hoping Ritchie had found a different way of doing things as his update on this character was touted to be far more vibrant and hands-on than the classic Holmes with whom I have no familiarity or loyalty besides. My other issue comes from the uber-bromance they try way too hard to force between Holmes and Jude Law's Watson, as it came off less "Tango & Cash" or "Lethal Weapon" more "Grumpy Old Man." In general, I found Law's Watson whiny and irritating and couldn't for the life of me figure out why a cool cat like Holmes would want to hang with him; the best bromances or buddy teamings come from two guys good at different things who compliment one another, as opposed to here where Holmes is such a clear alpha and Watson is just bitchy. This is one instance where I wish they had stuck with tradition and left Watson as the trusty sidekick rather than trying to ram home how invaluable he is to the point where he's the cool one with the chick and Holmes is the crazy shut-in who needs him. Rachel McAdams is great when utilized, but sadly that's not enough. Mark Strong makes a fairly impressive villain on how he carries himself, which makes it a shame that you never get any real sense of him being a legitimate threat from the plot. It may sound like I'm down on this movie a lot, and that comes part from going in with high hopes and part from not quite being able to figure out what was missing, but on the whole it was clever and fun, if not the homerun I was rooting for.
If you haven't already, peep Kiel's take on this flick.