Yeah, I'm normally allergic to mawkishness and I can certainly see how it could hit you that way - it strikes me as maybe the only legitimate complaint (other than simply "doesn't do it for me, not my kind of thing") that could be leveled against it. I think the reason it doesn't hit me that way, though, is that Charlie, as far as I recall, handled his crushing situation manfully and, by choice of narrative point of view, the story doesn't really try to "force us to feel" - it's all elliptical, implied feeling. It's told stoically, despite being such emotionally loaded material. The feelings are, for instance, the teacher for Charlie and Charlie for Algernon more than Charlie for himself (or, obviously, Algernon for himself
). But it is true that, if readers don't have an emotional reaction to it and they aren't directly fascinated by the experiment itself, that does knock out a huge chunk of the story and they aren't likely to like it much.
As I've said elsewhere, put me down as a "read the story and the book in that order and like them in that order" guy - I don't much care for the novel given that the story exists, and do give the story 5/5. But I'm sure I'd give the novel a 4 or 5 of 5 if the story didn't exist. And I agree with j.d. that, by my recollection, the story is pretty much left alone in the novel but is just inflated with some subplots and additional scenes - they're fundamentally similar.