If you like Guy Ritchie movies (Lock, Stock and Two Smoking Barrels; Snatch, etc) this one really departs from the standard Guy Ritchie format. Which is a good thing. If this was Snatch or Lock/Stock played out again with a pasted-on Holmes and Watson veneer it would have sucked.
But it didn't.
Not the Holmes/Watson you might be used to - if you grew up watching Basil Rathbone. But it works. Actually Downy's Holmes is believable as Holmes if you've read the books and Law's Watson is a little more kick-ass than one would expect. Thankfully - there's no "elementary my dear watsons" or deerstalker caps - and actually makes allusions to Holmes's drug addictions.
It's witty, has good character chemistry, a good story, technically shot well.
And the best thing - it really involves a mystery story with everything is logically explainable without supernatural or deus ex machina explanations. Although it is questionable whether the viewer could come up with the solution to everything based on what was shown. Now that I think about it, one would really have to have some obscure knowledge to put everything together. So yeah, some mysteries in the flick can't be solved by the viewer prior to being told the answer because you aren't given everything you need to know beforehand.
And yeah, there's a couple events that stretch one's suspension of disbelief, but not so much as to ruin a good rollicking fun movie experience. Good job by all on this flick.
Definitely entertaining and worth watching.