Yes, they are great! It's actually a pretty good way to introduce other people to philosophy as well, I find... LinuxFreeorDie is pretty great in general, too.
Oh, LinuxFreeorDie is Existential Comics' writer. Dead Philosophers in Heaven is also pretty great, though the punchlines are sometimes a bit cryptic and the art style is definitely... Unique. Plus, they aren't as frequent as Existential Comics.