So far, I am pretty new. I have leveled my SI to 34, JK to 28, Trooper to 20, BH to 20, and IA to 15.
SI - I am really enjoying the story. I feel like from the 'slave' alien origin I am not a follow-orders guy of the Empire but one rising to power and destroying everything in his path. Personally the end of Act 1 was awesome:
JK - Also enjoying this, but currently I'm more interested in Kira Carsen than the main story.
Trooper - I can see good things for the future of the Act 1 storyline, I am a neutral, based on what orders are and what I feel would be best for the Republic, so I make some bad decisions from time to time. I despise Aric Jorgan, though.
BH - Not fun for me, hated Mako, I knew the whole time that someone was actually 'whatever.' I think I played him wrong as a DS so I might reroll after some other classes and try LS.
IA - Enjoying it so far, playing LS so someone said before there was fun things to come. I think Jadus is interesting and I am confused what will happen with him in the future.
So far - SI > JK > IA > Trooper > BH
(I am sure this will change.)