I played all 8 classes, 2 of them twice. Although I have not even completed any of the chapter ones yet I have completed all of them till the end of the capital planet. Here are my impressions.
Sith Warrior : Korriban was interesting with a mini finale but the capital is forgetful and no mini-finale. Main antagonist is not clear at this time. I played the warrior as a pure light side which is quite interesting to say the least.
Sith Inquisitor : Korriban was similar to sith warrior but with a twist. Of all the force users, the lightsaber quest is a non-event. Capital planet ends with a mini-finale and a sort of mini-twist and some intrigue. Main antagonist is not clear at this time.
Agent : Hutta was quite boring. I guess many people likes it because Agents came with twists and turns. At least one twist each for Hutta and capital. Main antagonist is somewhat clear at this point in time.
Bounty Hunter : The twist came early in Hutta but also lacks emotional impact. Hutta itself is also average. The capital planet quests are however, very interesting. Ends with TWO mini-finale. Main antagonist is clear at level 3...
Jedi Consular : Tython is mainly forgettable but the lightsaber construction is a major portion and serves as a mini-finale. A small twist at the end of Tython. Capital planet quests revolves around solving the twist. And then after that more of the same which is why many people feel it is boring. Only interesting if you are the self-sacrificing, scholarly type. Main antagonist is slowly revealed after capital planet, making it an mystery.
Jedi Knight : Tython is similar to JC. You have a early antagonist and finale with light saber crafting. Depending on your light side/dark side choice, the outcome may be a bit different in terms of later set up. This is the only class I have not played the capital planet yet.
Smuggler : This has got to be the most funny class to play (at least as a male, jerk *** roguish type). I have chuckled at some of the dialogue choices I made myself. Story is very personal, with mini finale at the end of Ord Mantell and a sort of significant moment finale at the end of the capital planet with a surprise at the end. The surprise will then lead to a bit of boring storylines (but funny conversation). Good for players who don't want to care much of the conflict and who likes to role play the Malcolm Reynolds, pre-ANH Han Solo type. Main antagonist is revealed early, around level 1 or 2...
Trooper : The class with the closest link to the republic storyline. A big twist at the end of Ord Mantell with the reveal of what I think as the main antagonist. Capital planet quests are a follow up of the manure that hits the rotating blades. Best played as a heroic soldier.
Note : As mentioned, most classes are played till we get the starship. So the "main antagonists" I mentioned may not actually be the main ones if there are twists. So please don't spoil us.