The following contains significant spoilers for the Sith Inquistor storyline in The Old Republic.
I finished my third class storyline in The Old Republic: the Sith Inquisitor. By and large it was a very good story. However, it did have two major flaws.
The first flaw was the motivation behind the villain for Chapters 2 and 3, Darth Thanaton. He is presented as a arch-traditionalist who takes you in dislike for violating traditions (I think). However, I could never really understand exactly why he felt this way.
Your master, Lord Zash, uses you to kill her master, Darth Skotia, in Chapter 1. I think this is supposed to be the non-traditional move that Thanaton opposes. But Sith killing Sith is a normal state of affairs. The primary rule seems to be "don't get caught". Zash arranges things such that she has an ironclad alibi for Skotia's murder.
As far as I can tell, this is pretty much standard for Sith. I don't see what the tradition that got violated was. So I don't really know why Thanaton became the enemy. I think that Bioware did a poor job explaining the motivations of the main villain, and as a result, my attitude to him was more confused or bemused than anything else.
The second flaw is that there were two major strands in the Inquistor story, and they were not in balance. One strand was all about Force ghosts and ancient mystic secrets. The second strand was about building a power base. The force ghost strand becomes too dominant for much of the game, with the power base coming in second. Then at the very end there is a abrupt and jarring shift in focus to the power and political game.
I think either the Force ghost story should have been the sole focus of the Inquisitor story, or its importance should have been reduced slightly, and the power base story been expanded in scope.
Ignoring those two flaws, the story was very enjoyable. If you just accepted that Thanaton was the villain, or that the Force ghosts were not involved in the last bit, the story went very nicely. I actually was neutral in alignment, some light and some dark, and I think it worked out quite well. I started Light Side, but then some NPCs became annoying, and the [Shock] option became too tempting.
Mechanically, the Sorceror was a fun class. I played as a healer for the last 25 levels or so, and it was quite viable. I would run with a tank, Khem Val, let him gather everything up, and then DoT things up, or Force Storm for AoE, and keep healing Khem.
I did try using a DPS companion a few times. I found that though the DPS companion did more damage, I kept getting aggro from the other mobs in the group, and I had heal both myself and my companion. I found that to be more annoying than just keeping Khem healed up.
Now what story next? I am thinking Trooper, to get the last buff and to balance Imperial/Republic. I was thinking of going Dark Side, but I'm having a real hard time being the bad guy.