Although Apotheosis is "technically" a Total Conversion mod for Marathon/Aleph One engine, the quality and work put into it (graphics overhaul, new weapon/enemies + tweaks to existing ones to make gameplay punchier, full game's worth of levels) make it much more than just a mod for Marathon and more like an entirely professional indie game.
For anyone who is a fan of Boomer/retro shooters, I absolutely recommend to check it, even if you aren't a fan of original Marathon games. Like the og Marathon games, it is completely free to download and play and IMHO, its a lot more enjoyable than a good deal of retro FPSes out there.
Although its story is linked to Marathon games (it's story acts as a side story), you don't need to play Marathon to get into the story. And the story itself isn't all the necessary to enjoy it either. The gameplay makes up for it.
As for my specific experience with Apotheosis, I can say that the few gripes I had with original Marathon's gameplay are no longer an issue here. The levels here pretty complex looking, but yet pretty straightforward in terms of progression as I seldom got lost at where I needed to go. And the maps made great usage of incidental and setpiece based combat situations.
Another issue I had (with Marathon 2 specifically) was the over abundance of swimming and water based combat. Thankfully no boring water melee combat in Apotheosis. Instead Apotheosis uses the vaccum/oxygen mechanic to create arguably one of the most memorable levels in the form of "Sky Burial" where we carefully traverse in the dark semi-destroyed space station at low gravity.
About the only thing that comes close to being less than excellent is the storyline, which seems like a rehash of Marathon 2's story. But it's still well done overall.
A solid 5/5