There aren't all that many fantastical genre movies with a 19th century setting. Even less that are original enough not to involve Sherlock Holmes, Dracula or such. So I certainly give The Adventurer points for being a truly new addition to cinematic Gaslight Fantasy and Steampunk. So if you love the Victorian period as well, the aesthetic of this film has something to offer you.
Unfortunately as a whole it doesn't sit that well with me. It's so obviously based on a young adult novel that it's jarring to an older audience. There are family films capable of being loved by all ages, The Adventurer isn't one of these. The story isn't that strong and many characters behave cartoonishly. As the plot advanced I felt myself caring less and less.
Still I do not regret watching it. But if you do see it, manage your expectations.