Having completed all 8 stories, my personal opinion is that the storyline is alright, but I found it to be the second to last story in terms of how interesting it was. Mind you, I consider it leaps and bounds better than the last place storyline, which I consider to be horribly done(as anyone who has seen one of my SI rants can tell you).
As for why I rank it so low? I really can't give any kind of concrete reason for it. I just found it less interesting than the 6 stories that I rank above it. I guess I would say that I consider it competently done, but the other 6 are well done to amazing. To some extent, it just didn't draw me in. For example:
Then there is the Nadia romance....which is all kinds of hilarious for all the wrong reasons. As for the Rift Alliance, I found myself completely uninterested in them except for the guy from Balmorra(who leaves your ship way too early) and the two you recruit in Act 3. The others were just freaking annoying. Well maybe not Senator Grell, but I didn't care for him either.
I guess the bottom line for why it ranks so low is that I just didn't care about a lot of what was going on for one reason or another.
However I was playing a LS JC. I have heard that the storyline feels considerably worse if you play a DS JC. My understanding is that the game doesn't really react when the JC makes DS. You geefully kill off multiple masters of the Jedi order? Oh well, they weren't important anyways. So that disconnect may be part of why some people dislike the story so much.