As a bass player, he's purely average, maybe a little above, but does what he needs to do. I'm not going to flower it...based on musical ability alone, you could plug any competent bass player (or drummer for that matter) into U2 and would never know the difference. Sorry. In U2 at the End of the World they even entertain the idea of ditching him. Of course, that would seem unfathomable now.
Of course, Adam and Larry's contributions to U2 are in the intangibles, and I can't pretend to know what goes on in the studio between these four men. U2 certainly isn't U2 without those two, and the dynamic that they bring is something that can't be quantified by sheer musical talent. 30+ years of friendship, how they work together, play off each other, the energy in the studio, the catalyst those relationships are for the creative process, etc. count for quite a bit, and part of what makes U2 special is that four friends are making that music, and I believe it comes through.