I pre-ordered back when they first came up, had an absolute blast playing the beta weekend of Nov 11th. My guild has decided they are going to be on the Sith side so I played Galatic Republic during the beta so the story would be fresh when live hits, although I did play the mirror class, ( I'll be an agent , so I tried smuggler )
spent a few hours only trying out a Jedi as well, both classes felt evenly matched , although I only tried 10ish levels of each.
With regards to having 8 unique play throughs, while every class has their own class
story to follow, there are many side quests that overlap. For example , Smugglers and Troopers start on the same planet, so outside of the story quests, they have the same starter quests. and it appears all 4 Galatic Republic classes end up on Coruscant after 10 levels, so likely all of them will overlap on non story quests at that point.
( this was as far as I got, so it could be that Coruscant has so many quests that you can vary which quests you do on each alt, I'm not quite sure )
anyway, cant wait to play live )