Glad you like them, although I'm surprised — "bleak" doesn't even begin to describe some of the stuff in Banks' novels.
I don't know if you've read the ones I mentioned, but [SPOILERS AHEAD] in Consider Phlebas, every main character dies, the protagonist's species goes extinct in the process, and the epilogue essentially summarizes the whole story as accomplishing absolutely nothing; in Use of Weapons, the protagonist is revealed to have a past as a psychotic, genocidal warlord, and most of the wars he is subsequently involved with as a mercenary go badly for most involved; in Against a Dark Background, everyone except the protagonist dies, and it's fairly traumatic besides (though the unpublished epilogue, which is on the Banks web site, lifts it up a little). Another bleak one is Look to Windward, although the tone is more elegaic; the fate of that sorry anthropologist who studied giant floating aliens is one of the saddest scenes I've ever read (very much reminiscent of PKD's "Rautavaara's Case").
I agree that philosophically, he's on the positivist side of things, despite the amount of sheer death and destruction that goes on in his books. It's a trait, a sort of sarcastic, righteous rebelliousness in the face iniquity, that he shares with Terry Pratchett and Douglas Adams. (Speaking of, I found Feersum Endjinn to be a highly Pratchettian novel. It's also quite different from anything else Banks wrote. It's excellent and probably underrated.)
As an example of a polar opposite, I would point to Stanislaw Lem, who seemingly had zero faith in humanity. Solaris, His Master's Voice and so on take an extremely dim view of humankind's ability to do anything except navelgaze and quibble.