izigame.me

For its price and the content you get, it's a great expansion to a great game.
Pros:

The 4 JOTL classes are fun to play and feel distinct from the original 17 in the base game
Most scenarios are designed with these new classes in mind and can range in difficulty (more on this later)
With a narrower scope, the plot is more focused and improved over the base game.
The duration of the campaign feels just right: Not so long that you'll be occupied for months and not so short you feel like you're not getting your moneys worth

Cons:

The difficulty can vary whether or not you are familiar with Gloomhaven. With the board game, this makes sense as it was intended to be an introduction to the series, but I wish they balanced the few introductory missions around your current prosperity level (more on this later)
The monster variety can be considered a bit lacking. There are some unique designs but feels limited when compared to the base game.
Most of the new items are underwhelming. There are some nice unlockable ones but feel like they are tied to a specific character, rather than available for all.
The plot, while better, can feel a bit 'one note' in terms of narrative arc and expected conclusion.

Now the spoilery stuff

What the heck happened with the design of Tainted Blood? While most of the game's difficulty could be considered between easy - medium, with the occasional challenging mission, Tainted Blood can have some of the least forgiving, and cruelest expectations of the player. I play with 3 mercenaries and from what I have read on forums, this is the hardest number to play the scenario with. I did eventually figure out how to mitigate the spawning to a more managable level but I had to change up my preferred party (originally it was Hatchet, Red Guard, and Voidwarden) swapping the Voidwarden for a Demolitionist. What's worse, I had to rebuild my 'fun' Demolitionist just so it was more consistent for that one scenario. This would all be fine if even with the ideal strategy, it wasn't somewhat dependent on luck. Not only does it introduce an excess of enemies that you need to deal with, they also curse, muddle, and poison you, reducing the likelihood of landing your cruicial attacks and lowering your survivability. I wouldn't have minded if this was an optional mission post-campaign, designed to challenge you, but this is required to beat the expansion. And of course, once you beat this campaign, the remaining scenarios feel so drastically easy by comparison, it doesn't leave you with a sense of accomplishment similar to the base campaign. This isn't enough to make JOTL not worth playing, but it does beg the question of how often was this playtested and was it designed for Gloomhaven veterans or those new to the series? It's just a strange outlier that I'm sure caused a number of players to feel similar frustrations.
Also, in the strangest criticism ever, the new classes feel almost 'too good' like they have very few cons. The Red Guard can be designed as a tank or ranged AOE damage dealer, the Hatchet can pull out some crazy burst damage with excellent attack modifiers/status effects, the Voidwarden can literally do whatever she wants, whether hard support or status inflicting damage dealer, and the Demolitionist can be a fun stompy compliment to the Craigheart or a somewhat hybrid damage dealer with a consistent attack modifier deck.