Playtest with 5E
I recently finished the third ever playtest of Stuck in Jawbone, and it was doubly successful: the players seemed to enjoy it, and I found a few gaps I'd overlooked. This was also my first "beta" playtest, with a completely written draft of the module, and my first with experienced players.
But I'm going to go back to the first run-through.
It was the only one I've done in person, with some excellent local friends. I had only a very rough outline, and we were playing D&D 5E (a poor choice, for multiple reasons, though I found ways to work around some of the problems). I had never played 5E and never GM'd any game. Of the three other players, only one had played any TRPG at all. Considering all that, the game ran surprisingly smoothly, thanks to patient and good-humored players and willingness to just wing it when we weren't sure what the rules were. That said, I would not suggest anyone writing up a complicated mystery with a homebrew setting and a gazillion NPCs as their first GM experience.
One innovation I am proud of came from this first run, though. Because this group was much more into social information-gathering (and, let's be honest, flirting with everyone they met) than monster-fighting. By default, D&D gives XP for killing monsters. I was worried they'd all still be at Level 1 when they got to the end, and I did not want a TPK. So I came up with an alternative XP method: I made a list of secrets and awarded XP when they learned them. They were Level 3 by the time they got there, and that worked nicely. Well, except that I accidentally had an NPC deal the killing blow to the big bad. Turns out that's a risk of having the GM roll for monsters and NPCs that I hadn't anticipated.
It took 7 sessions, partly because we were all learning the rules, partly because I was still figuring out I needed to stop trying so hard to be subtle, and partly because when they actually did get into fights, they took SO LONG to resolve. D&D combat is sloooow.
I could probably run it a lot faster if I tried again with 5E. It's completely doable. But in between that playtest and the second one, over a year later, I discovered Dungeon World, which has been a much better fit both for the module and for my GM style, so I'm unlikely to go back.