For my own playthrough, I thought it was very good, at least based on the "decisions" I made. I haven't had a dark side playthrough yet, but I enjoyed the direction that the story went. It's more than we've gotten out of any individual SWTOR storyline so far, so I'd say that's an improvement, at least.
As for Arcann, I'm glad that you have the option to let him live. I was getting an impression that you actually aren't able to kill him, even if you do go dark side, that would be really dumb if that is in fact the case. I was fine with him saving his mother in the end, and we don't even know if he will be good or bad in the next expansion, since he didn't really have time to express his emotions on the subject while escaping from an exploding flagship.
I'm glad that all of the involved characters at least felt like they were dynamic characters with changing opinions, and with their own individual goals. The story is obviously incomplete, but it at least felt like a book ending in a trilogy, which is enough for me for now.
Ultimately I think that it was a satisfying conclusion to a satisfying story. I didn't really get the kill the emperor storyline I wanted when KOTFE started, but for what we got, I think it was pretty good.
The story, however, doesn't excuse the fact that we still haven't been given any context whatsoever. I would have had much stronger ties to the events within the story if I knew where all the Jedi and Sith went, and what the Republic and Empire were up to in the meantime.
It's obvious that Bioware has great writers working on this, but they're not investing the time into creating a setting to tell their story out of fear of writing themselves into a corner. It helps them out a lot, but leaves our entire community in shambles, scrounging through the scraps.