Rating: WARTY!
I positively reviewed a novelization of Shaw's Pygmalion back in January of 2015, but this is the actual play which I had never read. If your only exposure to this is from the 1960's movie My Fair Lady which starred Audrey Hepburn and Rex Harrison, then you may be surprised at how much they changed the story - particularly the ending, which actually ended-up subverting Shaw's intent.
The character of Eliza Doolittle was based on a woman with whom Shaw had an infatuation, and the story begins very much in line with the treatment the movie later gave to it, but there is no singing of course, and there is no trip to Ascot. Why they'd want to visit a place that makes water heaters I don't know, but...(that's a joke!). Eliza comes into some money (actually tossed into her 'flahr' basket by Henry Higgins, a voice coach and student of language. She hears him boast that he could pass her off as a duchess, and decides he can at least teach her to speak sufficiently well to find work in a flower shop. She visits him, and he and Colonel Pickering wager over Higgins's success or failure with this conversion project.
What's rather glossed-over in the movie is how dependent Higgins becomes upon Eliza to fetch and carry for him, and keep his appointments straight. What's completely glossed-over is how intelligent she is and how capable. In the movie she's rather made to look incompetent and slow, but in the play, she comes along quickly, and proves herself very capable - even picking up how to play the piano because she has such a good ear.
In the end, Higgins wins his bet, and inadvertently and rambunctiously sleights Eliza and her hard work. She leaves after an argument and Higgins, in a panic (he hasn't a clue what's going on without her to shore him up) he visits his mother and finds Eliza there, she having become quite friends with his mother. Instead of her showing-up at his home afterwards, and him relaxing into a chair and demanding his slippers, she bids him goodbye, and eventually marries Freddy. They make a go of their life together, difficult as it is, with a little bit of help from their friends, including Higgins and Pickering surprisingly.
The thing is that this part isn't part of the play! It's all tacked on in a sort of short story or epilogue at the end of the play. Frankly it's done rather amateurishly and looks like a kludge, but overall, the story is much better than the movie, much as I do like the movie and adore Hepburn's sterling role in it.