I love this book so much that I wrote my own version, so that pretty much tells you what I think of it. Orczy's novel is typical of its time (1905) in having far more description and "telling" than a twenty-first-century reader expects. But once that reader adjusts his/her focus, s/he will find this story of a swashbuckling hero who hides behind the persona of a foppish aristocrat riveting. Batman, Superman, Zorro: all the dual-identity superheroes get their start here, with Orczy's vision. And there is a love story, too, between Sir Percy Blakeney and his befuddled wife, who learns her husband's true character as the story unfolds. I read it first when I was 14, loved it then, revisited it many years later, and was struck anew by just how good the underlying story remains.
And if you want a modern update, without all the pages of exposition, try my "Not Exactly Scarlet Pimpernel"!