Saw Cursed Child a couple weeks ago and, sad to say, I found it to be pretty dreadful.
Yeah, there are some good effects, but the plot is nonsense and the script is horrible. Dialogue rarely rises above the most basic "Oh, look!" level, and there is no complexity to the characters at all. In fact, much of the play (which is far too long) seems to have no interest in doing anything but triggering fan reactions -- Hagrid! Floo powder! Trolley witch! Dumbledore! -- that are painfully cobbled onto the plot.
I've read all the books and seen the movies several times, and am quite surprised that some fans actually like this mess, as it distorts and violates several canonical aspects of the HP Universe. (Am even more surprised that J.K. Rowling signed off on it.)
A huge disappointment.
P.S. After seeing Cursed Child, I digitally checked the play out of my local library. I didn't find any significant difference between that "rehearsal script" and what was presented on stage. There may have been an altered line here or there, but nothing struck me as being particularly significant. Overall, I'd say that what I read was what I saw.