I've heard that before but have never seen it. I have been told by many after They Live it's downhill for his filmography.
They Live to me is between an 7 or 8 but last year when I watched it again it felt like a 7 to me, which in my ratings is good.
I would rank They Live as Carpenters 5th best/favorite film
Go ahead and watch it sherbro. Its a surprisingly creepy and weird film, but in a good way.
With regards to the subsequent Carpenter movies, I think even Escape From L.A. and Vampires are decent flicks and maybe even Village of the Damned. They only cost a couple of bucks on itunes.