I'm playing an Infernal Pact Warlock and I've really enjoyed it.
We are going through KotS and the Irontooth fight went fairly easily becaus eof my ability to run up to a group of enemies, kill all of the minions at the beginning of their turn, and lay down some hurt on the survivors. (Armor of Agathys is NICE)
I had enough HP to take the hits I took and I actually kept my HP at bloodied the whole time on purpose because I am a gnoll, and that ups my damage.
That said, Fey Pact has amazing mobility and Star Pact has some good control.