Depends what sort of PvP you want to do.
In a team, guardians are great (once you get the skills set), you're not going to be the one with the huge kill tally, but you will enable your team to win the day. Guarding others, force freezing and taunting the bad guys is satisfying.
If you want to go lone-wolf hunting Imps then (imo) you are looking at the wrong class. Or at least, I can't figure it out - seems to me that the ranged classes hold all the aces in that department.
If someone can explain to me how you're actually supposed to hold a ranged player inside melee range long enough to worry them I'd appreciate it, because right now my (L34 defense spec) guardian is a bundle of fail when it comes to Imp agents / Sith sorcs. You just can't get close enough for long enough and you can't do enough damage in the short time you have before they push you away. (I don't go looking for them, but they see a guardian solo and dive in for an easy kill)