I beat Twilight of Spirits and it was one of the most forgettable and boring games I ever forced myself through
I can see this, because its gameplay is far too easy, save for a handful of fights (and unfortunately, one of those fights is very early in the game). I do think the story is stellar though.
Arc 2 is probably the best overall, but it drags on and on at several points. I saw on the documentary disc that comes with the collection that players complained about the first one's length, so the designers made the second very repititious - stupid motherfucking players/designers >:( ! I still like it a lot though.
Arc 1 is fast and worth doing because you'll get to know characters that return in Arc 2. I find the gameplay fun after you have more than 3 characters on your team.
I agree with Tomara on Arc 3. It's good except for the lack of difficulty, the excessive amount of dialogue/text/fetch quests, and the redundancy of revisiting the guild after every task. My tip is to always do the highest numbered mission first - that way you'll beat it while doing as few fetch quests as possible. Oh yeah - the graphical change stunk too.
I'm also not a fan of the dungeon design used in Arc 3's final dungeon and optional dungeon, and Arc TotS final dungeon. I had to use a FAQ for all three of those. I've never been a fan of this dungeon design; it needs to be done away with forever or the player needs better hints like in Mario RPG's forest where you follow Geno.