IMHO Consider Phlebas and Use Of Weapons are the worst of the Culture series, neither of them are particularly good. The former sets a trend early on and doesn't deviate from it (it's like reading a slow motion train wreck), the latter fails to make the characters relatable and the end revelation is lame.

If you haven't, I recommend picking up some of the later books in the series. They can be read in any order. It's unfortunate mr. Banks isn't around to write any more, because it seemed that his work was showing considerable improvement towards the end.