Given that Sherlock Holmes is about the most filmed literary character of all time, it was always going to be a bit of a struggle to find too many new angles for a modern day fresh blockbuster movie to explore. And thus Warner Bros took a chance. It cast Robert Downey Jr in front of the camera, and put Guy Ritchie behind it. It's fair to say that it wins one gamble, and struggles more with the other.

The first half of the film is where it's at its weakest. Opening - if you discount the brief but fun cobblestone credits - with Sherlock Holmes being chased through the streets of London, we soon arrive at a fight scene that makes your heart fall through the floor instantly. Fast edits? Yep. Exaggerated bursts of speed? Yep. The general feeling that you're watching a videogame play out? You better believe it. For Ritchie spends much of the early part of the film priming Holmes as an action movie hero before he can get down to the true detective work.

This is not a wise move. We get a couple of scenes where Holmes explains in detail how he's going to take down his assailant, which he then does, before this whole idea is left alone for the second half of the film. If you're wondering what the point of such fighting segments was, you're not alone. It's as if Ritchie decided to chuck as much testosterone at the screen in the first half as he could, before someone had a quiet word in his ear. It feels like historical Fight Club at one point.

Fortunately, in time the plot comes along to jolly things along. Here, we meet Lord Blackthorn, who is hung and pronounced dead by Jude Law's Doctor Watson. Only shortly after, he's then reported as seen walking around alive and well, and his tomb doesn't quite contain what it should. Is this witchcraft? The occult? That's for Holmes and Watson to work out, and the film eventually allows them to do so. And after a fairly flat first half, things splutter into life perfectly well.

The masterstroke here, of course, is casting Robert Downey Jr in the title role. He makes much of his surprisingly slight dialogue, in a script that's quite low on humour and heavy on babble. He's aided in the film, of course, by Watson, but not helped particularly by Jude Law, whose character is rarely given much to do. Crucially, it never really feels like a double act at work, either, and not much convincing explanation is provided as to why Watson continues to follow Holmes round like a lap dog.

Rachel McAdams as Irene Adler doesn't fare much better. She disappears for most of the film, flitting in as Holmes' former love interest before turning up mainly for the final act.

That proves to be a wise move, too, as it's in the second half when Sherlock Holmes settles down and finally starts to enjoy itself. When Guy Ritchie focuses his camera on simply telling a detective story wrapped around solid action sequences (one of which the creators of Saw would be well advised to photocopy, and another, in a ship yard, is genuinely brilliant), the film pulls itself into three star territory.

Inevitably, the clues to the bigger mystery are contained within, but when Downey Jr is finally allowed to get down to solving the case, the potential of this new Holmes is unleashed. Even then, you have to offset a surprisingly underwritten villain (played perfectly well by Mark Strong), who has to battle to keep your attention when a greater foe is regularly hinted at.

It's a bit of a scattergun film. It asks you to sit through very modern moments of cinematic style before finally slowing things down to allow you to enjoy what you came for.

Ritchie, to be fair, pulls off a few strong moments, but rarely feels like the ideal choice to helm the material. His directorial style is way too showy at times, and while he's a director of talent, he's not a perfect match for good chunks of the picture.

Sherlock Holmes, nonetheless, is a sumptuous and clearly expensive production, with many scenes brimming with extras and glistening with detail. But it's inevitably its lead actor who drags it by the coat tails into a decent night out at the pictures. There's a natural charm and sense of fun to Robert Downey Jr that fits the role of Sherlock Holmes like a glove, able to switch on the serious when he needs to crack various facets of the case, while always remembering to gently have some fun with the proceedings.

As you might expect, the last few minutes are spent busily teasing a sequel, which wouldn't, in hindsight, be too bad a prospect. But for a Sherlock Holmes 2 to work, Guy Ritchie could use showing a bit more belief in the written material, and Jude Law and Rachel McAdams really need something more to do.

Above all, though, they need to get Downey Jr back. He's the best reason to watch this film, and you suspect he'll be the best reason to watch the next one too.

A bit more of a focus on the detective work wouldn't hurt, either....