Depends on what you're going for.
The Halo single player games are always fun, but none of them really had the magic that the original Halo CE had for me. I mostly played Halo for MP after Halo 1, and the campaigns were always something I'd play through but they werent the reason I picked up the games. Killzone is similar to that.
So, if you're expecting another "Halo 1 OMG MIND BLOWN" sort of revolutionary experience, well this isn't it.
But with that being said, it's an extremely slick shooter with great mechanics (not just gunplay, but also tactical elements), very impressive art direction and sound (the music is SOOOOO good, reminds me of blade runner).
So, if I were to compare it to recent Halo single player games like Reach, it's not quite as open and doesnt have co-op, but I'm having just as much fun, if not more. It's not just the next-gen shine here talking either.