I think to say they are both friendly is probably accurate. To say they have much in common is probably not. Yeah, they are both in bands that have been huge for 20 years, but I;ve heard Lars praise U2 before but have never heard or would expect to hear Bono or anyone in U2 to praise Metallica. I suspect U2 respects Metallica a lot and probably thinks their music is not of their taste.
They live in the same music world, I suppose, on different musical continents. It's easy as a heavy metal guy to respect rock albums, but if you are a rocker, it can be a stretch to respect Metal, especially with U2, they probably don't even get it. Bono said they were the world's loudest folk band, might be accurate.
Different worlds, different music, but i would guess mutual respect, certainly.
Plus, Metallica hit the mainstream around 91/92, after making a conscience effort to reach the masses. U2 were already huge and were deconsturcting their own sound at that time, perhaps seeing they could reach the masses by turning in on themselves.
Two different trains of thought, different avenues taken etc.
I did see James say on the Vh1 thing " I really want to see U2".
I'm sure it's a mutual respect and based on the fact that I've never heard U2, Bono or any of them comment on metallica, is probably verification that they respect them and don't like their music or else mouthy ass Bono would have mentioned them at one point or another, either in praise or derise.