By who? All Arsenal fans know they were great together.
I'd say it's underrated by the wider commentariat. Ask most people what the best central defensive partnership has been in PL history and they'd invariably say Adams-Keown, Ferdinand-Vidic or Terry-Carvalho. I'm not saying Toure-Campbell is better than those, but it's in the mix.
I'd also say it's a bit underrated by Arsenal fans actually, who I think for the most part would point to the partnerships of the 90's. Arsenal fans obviously don't underrate the Invincibles, far from it. But I think they underrate that particular component of the Invincibles.