I had, well, still have a 70 Swashi and I have all but shelved her to play my 70 Warlock. They are an absolute blast to play.
Group PvP is incredible. Like many of the above posters mentioned, having a warlock in your group is winning half the fight right there. Also, once people find out you are a Warlock you get the honor of being the first in line to catch an array of attacks from the opposing groups. At least smart pvpers anyway.
Soloing is a bit more difficult, as I am raid specced. I would loooove to get manashield but I don't want to give up that much DPS in a our raids. Focused casting is a must if you want to solo as a Warlock. If your target isn't dead or at least in the "red" after it has worn off then chances are you are a goner, generally speaking. Invest in Freedom potions and Health potions and obviously totems.
I am still working on my solo skills but boy are they a blast to play. I don't know of many people that would give up a raid equipped Swashi to re-roll as a clothie but I am glad I did.