I thought it was pretty good for what it was. The writing was a little more cutesy and superficial than I was expecting, and made it feel more like a simplistic Cliff Notes version of history than a real look back at the making of the show.
But David Bradley's performance was so strong and moving that I thought it somehow made the movie work anyway. It was hard not to feel for him as he struggled to concentrate or remember his lines towards the end, and there were moments where he sounded so much like Hartnell that it gave me chills.
As for the final cameo, my only complaint is it looked a little too
much like the actor was inserted digitally after the fact, instead of actually being filmed on the set. Which just made the whole thing look a bit cheesier than it should have for my taste.