Very rare, very obscure, so it's very unlikely you'll even come across this game. Worth it if you can find it.
Unlike most other Doom games, Master Levels for Doom II doesn't have a story. Actually, the other Doom games didn't really have stories either, but they had occasional narratives to explain some of the events. Master Levels for Doom II features none. Instead you just hop right in and start blasting. I suppose its intention was to be added to Doom II's story, which would explain the addition of nothing to the plot.
Some of the levels are really fun. I like the one where you ride an elevator and stop from floor to floor to kill enemies or solve switch puzzles. Also the level Vesperas has lots of enemy blasting, and that's always lots of fun. However one in particular is called Mephisto, and I don't like this one much. It took forever for me to beat it and it's packed with Revenants, which any Doom player knows are annoying in groups. Since this is basically an expansion pack, you'll need Doom II to play these.
The graphics and music are lifted entirely from Doom II. Some new textures are here, and they're pretty cool (like snakes on the floor). There's little more to say, since Doom's pixels are simple but pretty easy on the eyes and Doom II's music in particular is very grim and foreboding.
Not all of these levels are a blast. And Master Levels for Doom II isn't perfect. At times, the challenge seems overwhelming. Hell, finding the game itself is a challenge. If you find it, though, there's bound to be something in Master Levels that you'll like.