I’m afraid I have to dissent here. I disliked the first book immensely, in part because of the rape, but also because I didn’t find the book as a whole particularly funny. I tried to read the second one, and never finished - it just never gripped me. I think the series only works if you find it funny. Since I didn’t think it funny, there wasn’t any point in reading any more of it.
I’d put Flashman and Fraser in the same category of “Why do the best characters come from the worst writers?” as Tarzan, Holmes and Bond. Technically, Fraser didn’t create Flashman, but if he’d called him by a different name than a Thomas Hardy villain, the impact would have been about the same.
I’m one of those readers who dropped it after the first book.
Thomas Hughes wrote Tom Brown’s School Days, not Thomas Hardy: although the thought of Flashy spicing up the melancholy introspection of Flash The Obscure or Flash Of the Durbervilles with some rogering and knavery does have a perverse appeal. Flashman was actually quite a minor character in Tom Brown, as an all-purpose bullying sot: Flashy’s apoplectic reaction on reading the book and seeing the dirt dished on himself is a priceless moment in the books.
Fraser’s genius was in recognising that a relatively minor villain was by far the most interesting character of the original book, and casting him as a Hero Of Empire. There was a BBC TV adaptation of Tom Brown fairly recently, which was quite good, although definitely post Fraser’s Flashman: the young Flashy gets the meatiest part, and all the best lines.