I just rescued her, and initially it's pretty good I think. Likely because you have to catch up. But her opinions about the other characters etc, is done well also in comparison to the origin characters.
Sadly it looks like generally speaking, wether you take Minthara or not, an evil playthrough is just not as good as a "normal" one. Even if you are not playing a murderhobo, you kill off too many important side characters (and companions) while there is at least until act 3 no real other evil faction you could gain as an alternative ally (as you are still in opposition to the Absolute). The start of act 2 is really good though, I found that better than playing on the good side. For most things in this game, because it's so fast and has so many options, if you really like thing, you'll feel there is not enough of it. Mine is lack of specific content for high elves for example. Drow is in a different ball park altogether for example, they have more specific dialogue and flavor in the grove alone than high elves have in the entire game.
I doubt they'll be able to "fix" all of that. And maybe they just want you to not be able to get everything in one or two playthroughs. I hope though they can add little bit more for the evil side in the future.
