I went to see the Klein/Moriyama yesterday and I sort of agree with the above. I wasn't familiar with Klein beforehand but I was stunned and inspired by his half of the exhibition, and will definitely be looking up more of his work.
I thought I would like the Moriyama more, but ended up thinking that his early stuff there was too heavily abstract for me. I'm fine with abstract art in general, but something didn't click... it felt like, you know, random pictures of stuff, few of them really interesting. I got the feeling that a gallery might not be the best place to view it, as it encourages you to look at pictures as individual pieces, as opposed to in a book/collection way where you might find it easier to see a coherent aesthetic thread. On the other hand there are some gems in the later work, of which there is a lot.
It's a huge exhibition by the way. Finishes on 20th Jan so there's some time left to go.