I have a blast PvPing with my Sentinel. I use the Watchman Skill tree which many say isn't good for PvP, but so far it has done great for me. I genearlly shut Sorcs down since I can use force leap from 0-30m which is nice since it is an interupt. plus, Force Kick cool down is reduced to 6 seconds which also helps shut down the longer casts of sorcs, or the channeled force lightning. Against other classes I do fairly well. The interupt ability of Watchman plus the Dots do wonders. If i can quickly stack my dots, even if they CC me, they are taking damage from me.... and if I can stack centeirng fast enough (whcih I build 4 per attack that uses focus plus 2 each time I'm attacked) I can extend my HP pool through self healing; this plus a PvP med pac and expertice stim allowed me to take on 2 IMPs on my own in a warzone to secure a turret. I fought a Sorc and a marauder.
In open world PvP, I love my sentinel... I am tempted to try the combat tree for it's burst damage, but I love the constant damage and interupt ability of the watchman. I think that the ability to disrupt the enemy and do really good damage is better than not being able to shut them down as effectively and do great damage.