I haven't finished a playthrough enough to give a full review, but I just beat The Library and the mod so far is fantastic.
It makes a number of both subtle and obvious tweaks. It adds numerous new lore friendly enemy types, which is especially welcome with the flood as it makes them less of a boring slog to fight. It adds ODSTs alongside the normal marines. I really like the ODST visual which is at home with CE.
It adds new weapons to the CE campaign like a flame thrower (although to be honest I rarely use it due to how fast it goes through ammo) and sentinel energy beam guns that can be used. It even brings the covenant energy sword to CE.
The library didn't feel nearly as bad as vanilla, with a lot of tweaks given to it.
There are invisible changes too like the assault rifle's initial spread being way reduced, making it actually usable at midrange with short bursts. Hunters have been tweaked so that one pistol shot to their orange bit doesn't kill them. Warhog handling is better, with it accelerating much faster, and marines will now even drive unoccupied warthogs to follow you.
Big recommend of this mod for a replay of CE, it's available on the Steam Workshop page.
I really don't get the hate for the Library. It's one of my favourite levels in CE. The Flood are a creepy and scary new enemy (on the first play-through at least..) who back you into a corner and attack relentlessly. You're left fighting for your life with no way to retreat or charge past. It's one of the rare times in the whole series where you're forced to play defensively.
It's also the level that introduces rocket launcher flood, wtf is up with them? So unfair that the enemies get such a good weapon!
It is tedious, with repeated samey layout and a limited selection of flood enemy types. The mod mixes up the environment and adds more flood enemy types for variety.
I do, it's copy paste. A lot of the game is.