A tough recommendation - A Map pack with high quality, super challenging content.
Master Levels for Doom II is hard to recommend to anybody but the biggest fans of the original DOOM games that have completed them fully.
The Master Levels are a strange beast as far as game expansions go. Ultimate Doom and Doom II are full campaigns in their own right. Thy Flesh consumed for Ultimate Doom and No Rest for the Living feel like a new, solid, complete episode for their corresponding game. A campaign Master Levels is not. There is a reason why they are called Master Levels - having more difficult levels than those seen in the original game. This thought process was even used for the recently released Doom Eternal.
Master Levels is a compilation of the more higher quality levels from the Doom fan community. They are literally an unconnected set of maps (with a few exceptions). id selling this map pack basically feels like a bootleg of stolen, loose map files from the Internet being sold for profit and the user experience is pretty terrible.
Like the previous two games sold on Steam, you're given a lackluster, almost lazy feeling Dosbox port with no modern amenities what so ever, as if the games expected that they were still being played in the 90s. At least with Doom I and II, you were given the classic experience of booting up DOOM in DOS and thus forcing yourself to play it in an awful resolution and keyboard only controls, as vertical movement on the mouse moves you forward and back. Here, you're presented with an interface called DOOM-IT, that literally looks like interfaces I've dealt with from the late 80's/early 90s and my game crashed seconds after the first level load.
DOOM-IT is an application that runs in a DosBox emulator, essentially running an old looking DOS Program with an interface you can use to INDIVIDUALLY play a single map out of the 20 provided one at a time. Many maps are built as replacements for Map 01, so once you complete it, you're loaded into Doom II Map02: Underhalls, as if you were playing the original campaign. Meaning, if you want to play the other maps, you need to close Doom II and go back to the DOOM-IT interface. If you were to load the map into a source port, then you have to manually load the map from console commands - yes, I had to press Tilde, type the command "Map Map25" and press enter for the game to load the Black Tower Map. This is all provided you can get into it, as the first time I played it, the game crashed with in seconds and when I tried to launch it again for this review, Dosbox failed to work, even after a reinstall and I had to run it in Compatibility mode to get it working again. Thankfully, some nice person took it upon themselves to make the experience more user friendly and made a mod that creates an in game menu so you don't have to relaunch the game, if you're willing to go through the extra set up.
Imagine, after buying a retail game, having to download an unfamiliar archive file from a unknown Internet source, convert it to a .zip, duplicate one of the map files from the game, rename the copy, load the files into a "lump management tool" (a program that lets you edit Doom WAD files), edit the two copies by LITERALLY DELETING THE ALTERNATE MAP DATA FROM EACH FILE BECAUSE ONE OF THEM IS A SECRET LEVEL FOR THE OTHER AND THUS WILL NOT WORK PROPERLY IF YOU LOAD THEM BY DEFAULT, then package all your maps including the newly edited ones into the maps folder of the third party downloaded zip and after a convoluted process, hope you can load it properly into the source port/launcher of your choice. The actual process itself is not that difficult thanks to the creator's included easy instructions (though, why I had to EXTRACT THE INSTRUCTIONS FROM THE MOD ITSELF instead of it being included in the downloaded archive is very strange), but the fact that you'd have to go through all these steps to make the game reasonably playable is inexcusable, and no layman or console gamer would dare put up with this madness for an iD game purchased off the Steam store.
"So if I'm able to get past the interface and setup, is Master levels still worth a purchase?"
The Master Levels are high quality, but many if not most or all of the maps are confusing in layout and cheap with enemy placement. The Master Levels expect you to have a great understanding of the game's mechanics and map designs in order to succeed. Lengthy treks of backtracking, strange platforming segments, bouts of wandering aimlessly to find the next way to progress, unfair enemy placements, especially on harder difficulties where enemies can surround you in tight spaces or literally teleport behind you. I was looking forward to a challenge after fully completing the previous Dooms, but many of the levels had me more irritated than entertained. Part of that was my mod selection, that affected monster behavior, making them harder, but many of the maps had me both lost, confused, fatigued, and irritated. I can't call these poorly made because they are clearly designed with time and care, but these were maps made by people who clearly played loads of DOOM and wanted to impose their view of a real challenge onto players. I may personally lower the difficulty and change my mod set up to enjoy the rest of the maps, but if I find myself confused for too long like I did with Titan Manor and Garrison, I may just delete these .wads from my mod collection. These designs are interesting, but not exactly fun for many DOOM players.