I'm assuming we're talking about the 1968 film adaptation here, which, curiously enough, I've just finished watching (having read the book less than a year ago as one of Hoopy's excellent recommendations).
Overall I thought it was a good film with a strong performance from Cliff Robertson, and while I understand that matching the nuances of Charly's post-op progression with the book would've been nigh on impossible (especially as the book details all of it from Charly's POV) I was disappointed with the ending, which entirely cuts out his regression. If nothing else, it makes me appreciate that despite being a short book, Keyes worked in a great deal of character and information.
If you loved the film Parson, I'd really like to hear your thoughts on the novel, which I hold to be substantially more poignant. I'm sure you wouldn't regret it.
In fact, I'm thinking I'll be re-reading it myself in the coming weeks.