I wouldn't call this a good film but I found it to be charming in an amateurish way. It's rather like watching 1960's Star Trek or Doctor Who with modern-day eyes - it's corny and the special effects aren't great but it can be entertaining if you know you're not watching modern-day entertainment.
The production values, dialogue and direction aren't great and there isn't much in the way of dramatic acting until the climax of the film - the actor playing Holmes is particularly un-dramatic and speaks too softly for a leading man - but both Holmes and Watson are charming in their own way and have a playful chemistry together. Gareth David-Lloyd makes a sweet but quiet Watson who's a bit slow as times (though he gets to help save the day in small ways) and Dominic Keating isn't used all that much until the final 30 minutes of the film but he gives the strongest performance of all the actors involved.
The story wasn't too bad if you don't think about it too much - the bad guy (partly out of revenge) wants to use steam punk monsters to wreak havoc on London - but it is over-the-top at times (especially the part involving a hot-air ballon) and I wouldn't buy this film for the story alone. I have to say that I understood the story more on second viewing.
So overall, I wouldn't advise people to buy this film if they're looking for a professional movie to watch but if you're in the mood to watch something silly with friends that involves Sherlock Holmes, mechanical monsters and a cheap 19th century backdrop (and you don't mind films that have a cheap feel to them) give this a go.
For a mock-buster film, I'd give this 6 out of 10. For a film in general, I'd give it 3 out of 10.