I did a respec when I got my KE specialization. I'm really enjoying it. With several passives and actives in the inferno tree, I can cast so many spells thanks to spirit blade. It's crazy powerful. Also, don't really need barriers. I have Dorian in my party which starts a fight by casting static cage, my inquisitor casts fire mine at the center and then teleports to it and immediately uses fade cloak.
Mobs trying to attack Dorian will be teleported to the center with a fade cloak blast and fire mine waiting for them. Kills most mobs outright if you manage to pull it off and gives you full barriers.
I guess it's subjective? I'm having way more fun right now as a KE.
They can be fun, but like Bigdawg13 said, you eventually realize you are not going to die. They can do nice damage with the fire trap and fire barrier buff that empowers your fire spells. Basically means you throw up your barrier, consume half of it to empower your fire trap, then fire trap explodes and maxes out your barrier again. At higher levels you end up with like 12,000 barrier at all times, its quite insane, if not a little funny. You just destroy everything.
To be honest though you can pretty much do this with any class though on nightmare if set up correctly.
In the end You should play whatever class you feel like you would enjoy, what do you see yourself like in your minds eye? Often this will make a class more appealing to you if its more of what you feel like you would enjoy.