This is a difficult discussion to have at this very moment. As it is, F2P in SWTOR is in a state of flux, and they are slowly granting the Preferred class of players more for having spent money. Essentially, a Gold Membership still have the best deal, if you are going to be doing EVERYTHING in SWTOR, otherwise, you can piecemeal yourself the aspects of the game that you want.
The issue is that for a while, if you weren't Gold, you might as well not even bother. But they are changing that. As a note, I spent $5, to get Preferred status. That means I get several bonuses, as well as unlocking some cosmetic functionality (legacy name display, for example).
Things I don't like in the game, are things like, having to unlock the ability to wear Purple gear (unnecessary as it may be, it limits what I can get from the game). As well as unlocks for accessing certain gear. Even your partner's skin requires an unlock to use, and to even get at one point! That's right, there are rewards you cannot pick up unless access to it is unlocked. And you can't even unlock it later to use in most cases!
That aside, they have quite a few cosmetic items, which I'd be interested in, if I invest enough into the game to get to level 50. The game is quite playable for free, but I suggest dropping at least $5, even early on, to get the preferred member status.
Also, though the limit is only 2 characters, you can make 2 characters per server, so it is quite possible to negotiate all the storylines. As far as I can tell, members to go from Gold to F2P will keep their characters, but only 2 can have items bought for them. I would need further testing though, and I just didn't have the money to test it out.
If you like the game, I suggest getting the $5 bonus pack they sell in the cartel store (gives a lot of little trinkets, a ride, and gives you preferred status).