I saw this on Tuesday and would say I liked it on the whole and found the story quite moving (if a little one-sided). But this has the curious phenomenon where I think all parts of the show were executed quite well EXCEPT the writing. Others have talked about the amount of exposition and direct address the show has, and I don't think that would necessarily be a problem but they don't seem to want to lean into it or make it a key point of the story--in fact, it's all but dropped in the second act--so it ends up feeling like lazy writing. I haven't read Jacob Dunne's book, but I imagine a lot of the play was taken point for point from that.
I have to give props to Adam Penford for making the staging kinetic and interesting to watch, even with the amount of narration, though the pacing overall was a bit inconsistent. The asides in the first act where we see David and Joan react to their son's hospitalization feel incredibly inert in comparison to the rest of the story unfolding around them, and while I think the intention is to go for a moment of quiet emotion in the midst of Jacob's frenetic reenactment, it completely fell flat for me (maybe due to the lack of chemistry between the two characters, maybe due to Victoria Clark's terrible accent). But overall I found the story itself quite engaging to watch, as I was unfamiliar with it before.
Will Harrison is indeed a revelation (I was shocked to find out he wasn't British), and I hope he'll receive a Tony nomination for the role. I did get pretty emotional at his redemption story in the end, but there were parts of the show where I wish the characters had a little more time to breathe. It would have been nice seeing the bulk of the show focused on his time after prison, rather than that all being allocated to the second act. And I never quite understood what led him to turning over a new leaf and even being interested in restorative justice since it seemed like he fell back in with his old crowd again afterwards.
In the playbill there's a tribute to James Hodgkinson which surprised me since this seems like Jacob Dunne's story through and through, and James is more of a plot device rather than a feature focus. I did still ultimately enjoy the play and I'd probably recommend people to see it for the story itself, but I can't say that it's a "good play" necessarily.