So far, I have 4 characters: a level 29 Sith Inquisitor (Assassin), a level 19 Jedi Knight (Guardian), a level 18 Imperial Agent (Sniper) and a level 3 Bounty Hunter I just made earlier today.
I'm loving the the SI storyline so far; great story plus plenty of opportunities to make people who annoy you scream and cower. This is definitely the class for you if you want to feel like a bad-*** dark lord of the sith. Combat with the SI is I think best summed up with the line Dave Grohl says as Satan in the Tenacious D movie - "Taste my lightning, ****-ers!"
Khem Val is also awesome, though he tends to die a lot, at least for me.
IA has been surprisingly interesting and fun so far. Sniping is fun, and I love Kaliyo's personality. She actually reminds me of Jack/Subject Zero from Mass Effect 2, only without the angst and half the psycho. Storyline is interesting and really enjoyable if you love spy thrillers, espionage and intrigue (which I do, immensely).
JK has been enjoyable so far (just got to Taris, so still quite a way to go in the story yet). As others have said, playing light-sided makes it a bit cliched, but I barely even notice because it's still a story I love, and I always enjoy feeling like a bad-*** hero, but also the good guy who takes the moral high road. I feel like playing the opposite plenty of times, too, which is why I have my SI
BH I've just barely begun, as I said above, so I can't really comment just yet. I'm liking what I'm seeing so far, though, and from what the majority of other posters here have said, it's one of the better storylines with one of the best sets of companions, so I'm really looking forward to seeing what's in store
Just my two cents.