Pretty great! A large part of the experience here is from the visuals and setpieces, granted, but there's nothing wrong with that. Really this is one of the best scifi themings in a Quake map.
I think the map largely avoids an issue I remember from some 2000s-era "looker" maps ... overlarge proportions and simple layouts, places that felt like giant boring box-corridor-box sequences. This map is generally more interesting to get around in than those. The other end of that design spectrum is when a map introduces too many visual greeblies to snag your movement in smaller spaces; there's some of that here, but it didn't annoy me too much.
I liked that the weapon set was pretty tame, only adding the laser gun. One of the things I didn't enjoy about previous plays with the Keep mod was juggling a huge set of weapons with dubious reasons to choose one over the other. Not a problem here.
On the other hand the Keep mod is used to good effect here in populating a couple of distinct monster factions. It's far from a new thing to have two enemy factions that are also hostile to each other, but it's a lot of fun here! Cool to watch some fights work themselves out, and then entertaining to jump in and clean up.
I didn't have any issues with the progression, but I suspect I may have gotten a bit lucky. I ran across the final door while doing some extra poking-around in an out-of-the-way spot, so that when I found the relevant key much later it was only a minor puzzle to find my way back. If I hadn't previously found that door, it would have been a real head-scratcher.
I'm sure that with a big mod like Keep and complex architecture like this, there are bound to be bugs and jankiness. I didn't happen to run across much of that FWIW. As mentioned in the comments thread there is one spoiler-ish thing that I found can go wrong when loading a save, although noclip will easily recover from that particular issue. I do still need to figure out how to go _down_ a ladder in this mod though.
Finally, I know there's some discussion in the comments thread about how to properly launch and run this map. I can't blame anyone who is getting frustrated over that. But FWIW, here's what worked for me:
* Ironwail 0.8.0 (in my case, running on Linux)
* Mjölnir installed in the "mjolnir" mod folder, including its "Patch 1"
* This release installed in the "Marooned" mod folder
* Launching Ironwail with the arguments: -game mjolnir -game Marooned