Well this film is that level of fun. After kicking into gear they quickly take an old and familiar setting and bring it into the modern day (figuratively that is, since this is actually set in the late 19th century). They manage to do this without making it too youth oriented. The true fun in this film will be in rewatching it, the quick banter and fun plot twists that turn out to be thought out well in advance (for the most part), will only be more enjoyable upon multiple viewings. The layers, much like those found in the Pirates script, are multiple and cerebral. They hit the viewer without our full awareness and when we later realize this it brings a smirk if not a smile to the face. Many plot points that seem by chance or appear to be toss away jokes end up returning and playing a larger role in the story, it's nice to see some effort in a big budget blockbuster - especially when its a sequel.
The action and staging of set pieces might not properly belong in the era portrayed (according to some critics) but it makes for a fun and well balanced film in my opinion. This isn't meant to be historically accurate, its meant to be a version of history much like Gladiator or Pirates. The performances are excellent and I found RDJ to be even more comfortable in his role as Holmes, surprising since he has so many other films and franchises now on his plate. Having laid the groundwork in the first film he spends more of the film flexing Holme's emotional levels, mainly regarding his relations to Watson and Irene Adler. Jude Law becomes much more familiar as Watson and thus becomes much more likable because this time he adheres to the objective with much less resistance (possibly because he has a chance to really let out his frustrations with Holmes early on). The two obviously have that strong relationship that goes beyond friendship, something Ritchie quite enjoys playing with but that makes for something different and refreshing while allowing for moments of strong humor.
Also compelling was Noomi Rapace, filling a space left by McAdams departure after a short but meaningful cameo early in the film. Noomi is given somewhat of a small role and largely facilitates plot purposes but her character does have an arc and emotional stakes that make her important to the story, if not always fully utilized. She of course develops an interest in Holmes but again this is well justified given the events that transpire and doesn't feel as forced as one would assume. Another new face is Jared Harris, who is a very cerebral Moriarty. I really enjoyed his performance but was prepped for it, having enjoyed him on the series Mad Men. For those not familiar with his "proper/formal" Englishman style I'm not sure if he will come across as menacing as some might hope or expect, even based on the first film. I thought he played the villain in a unique way and did so quite well.
In short, if you enjoyed the first you will enjoy this, at least I don't see how you couldn't. If you didn't enjoy the first because of the occult vs. science storyline, the pacing, or perhaps the uneven still developing characters, I will say I personally found this one to be much improved and much more comfortable with itself all around. I did enjoy the first but never thought I'd need to see it again, but I found myself rewatching it after seeing this sequel, and this film I want to own and rewatch, again and again, just as I do with Pirates Curse of the Black Pearl.