well i thought it was pretty fun when it came out, but just got busy and canceled my subscription. and now i see there is cartel coins.
if u have subscription does it give u advantage to have em?
in ur opinion is it worth playing and subscribing to?
what is legacy?

Yes.
Yes.
http://www.swtor.com/info/legacy It gives you buffs and perks based on accomplishments in game, and some you can buy with credits.