I decided to give Marathon a try, since it's free now and I need a new game to waste my time on.

So far, it's pretty good. Only complaint is that everywhere pretty much looks the same as everywhere else, so it can be hard to find your way around, but that's not the game's fault, really, just a by-product of its age. I spent endless hours playing Descent as a kid, and now I'm appreciating the memory-mapping skills that developed. Finding it easier to ignore the game's map and just rely on my own sense of direction. So far, hasn't let me down.
It even managed to give me a bit of a scare.

In the first area, was wandering around in some ventilation ducts and a S'pht (or however you spell it) appeared around a corner and killed me. That startled me. Just goes to show that advanced graphics aren't necessary for immersion (though I'll admit they help).