Excession is basically Mind-porn. It's the one you read once you have some understanding of how the Culture operates and then by god it's good. I imagine people who read it early on are disappointed or annoyed by all the jargon - the very beginning in particular, even though it does get explained simply to you later on in the book.
Curiously, this is the first one I read - the premise sounded interesting when reading the back in an airport bookshop. It was a little hurdle to get past the beginning, but once adjusted very rewarding. Easier to read than Feersum Endjinn, at least!
Personally I think "Look to Windward" is probably the best general introduction to the culture.
A Feersum Endjinn reading tip/hack: from what I recall (having done it, years ago), you can save time (and annoyance) by simply skipping the chapters with the phonetically-spelled journal of the mentally-challenged character, Bascule the Teller, and the story still seems to work just fine.
You'll just want to remember that Bascule exists, though, so when he shows up -- way later, in the third-person -- you'll know who it is.
