This pack is very much essential for the complete Borderlands 2 experience which is why I recommend it, however I still resent the fact that 2K did NOT include it in the GotY edition. It's more levels which means it's mandatory when playing public games as playing a 61 with a bunch of 72's is not fun.
Pros & cons;
+ Extends playtime and enhances replay value
+ Adds one of the most fun raids in the entire game
+Provides more challenge as the enemies will still scale to your level
- Will cost you more money, even if you own the GotY edition that should have included this in the first place
One con, but a pretty big one imho. 2K should put this pack and the headhunters (and the... *shudders* skin packs I guess) into a bundle with a decent price drop. I know I won't be buying a single skin or headhunter pack and neither should you, it supports this kind of godawful marketing strategy and will doublessly spawn even more games that try to imitate it (see also the Call of Duty franchise).
It's not even the first time they've sold us huge but incomplete DLC bundles for this title, take a gander at the so called season pass if you will. Not sure about you, but I don't like being duped when it comes to content.
Buy it if you love the game to bits and have the time to sink into capping out several characters. Don't buy it with the game unless you know you will get a lot of mileage out of it in the long term.