Great fun -- these are my kind of levels, where a lot of thought has gone into the layout, and there are lots of things to tinker with. The monsters are kind-of low-key (most are possessed marines or sergeants), which I like a lot. The shotgun saw the most action. The levels are not hard (at the level I played it at), the challenge is in solving the puzzles to get to the end.
I liked the first episode of Doom best -- the later episodes have too large levels, that sprawl all over the place, with lots of tough monsters behind every corner. I'd rather not run around with both guns blazing, but explore the level and think about where to go next. The Serenity levels certainly allow you to do that.
At each level, a letter is part of the map. At the first level it's an 'S', the second level has an 'E', etcetera -- all the way to spell 'SERENITY'. That's a neat little detail, and these levels are full of neat little details -- but there's not much serene about 'em, so it's a total misnomer. ;)