After reading so many good things about it, I finally gave Shadowdark a try a few months ago. Our regular (mostly every other week) 5e game was on hold due to scheduling issues, so I proposed running a Shadowdark game as an alternative until everyone's schedules lined up again.
I'm not sure if I want to go back to 5e now. I've been having more fun DMing Shadowdark than I have in years.
Back when I was running a 4e game and getting burned out on the rules heaviness and long battles, I ran a couple of one-shots using B/X or AD&D rules. I remember loving how much faster the old games played, but the clunkiness of many of the game mechanics were difficult to overlook. The D&D Next playset looked like it was heading back towards a much more rules-lite game than 3e or 4e. And to be fair, the final 5e is still much more to my liking than either 3e or 4e.
But I think that Shadowdark might just be my all-time favorite version of D&D. Speaking as a DM, at least. It absolutely recaptures that feeling of the D&D that I grew up with (BECMI and AD&D) but through the lens of the last 30 years of game design.
Improvising is easy again; I found 3e and later to be much more difficult to run smoothly without a lot of prep time. I absolutely love the Shadowdark monster statblocks, where the majority are of B/X complexity with a single special trait or ability. Easy to parse at a glance.
Honestly, I can't say enough good things about it. Everyone in my group has been having a great time with it.