Expansion is good. It adds skill trees and other misc items that are of benefit to the base game and other expansions.
Skills are slow at first because it's effectively level 99->100 progression but they speed up once you get into the expansion, allocate skill points towards increased XP gain, and gain items/mastery. The more work you do before you go into the expansion will pay off as a lot of the items and buffs will help in small ways.

As mentioned in the store page, you must beat the base game boss to enter the Abyss.

If you load an outdated mod your game may break in unexpected ways. There is a huge disclaimer at the top of the page telling people this but they still load them and get the NaN error. Despite it not being a base game issue, the developer provided a way to fix the issue.

Expansion is good. It adds skill trees and other misc items that are of benefit to the base game and other expansions. Skills are slow at first because it's effectively level 99->100 progression but they speed up once you get into the expansion, allocate skill points towards increased XP gain, and gain items/mastery. The more work you do before you go into the expansion will pay off as a lot of the items and buffs will help in small ways. As mentioned in the store page, you must beat the base game boss to enter the Abyss. If you load an outdated mod your game may break in unexpected ways. There is a huge disclaimer at the top of the page telling people this but they still load them and get the NaN error. Despite it not being a base game issue, the developer provided a way to fix the issue.