I think the problem with Flashman is he is too black and white. Just as it's annoying when a character is predictably good all the time, it's equally annoying to me when a character is consistently bad/lucky. Let's face it, there's never any doubt about the ending of Flashman, from the very beginning of the book you know how he'll turn out. There's no tension when he is taken prisoner, no character development. His personal plot is simply very poor.
On the other hand, the atmosphere of the books is great, and he really immerses you in the world that Flashman lives in. It's just Flashman himself that I found a bore.
Bookmarks