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.
"Use of Weapons" also got the dubious honour of being the best SF book never filmed in a poll in the Register a few years back.
Come on Hollywood, you need to make a Culture movie - I believe Banks said he'd be OK with Consider Phlebas getting a happy ending for a movie (OK apart from the 851 billion deaths, and the destruction of all of those ships, planets, orbitals, rings and stars).
I've wanted to see the escape of the Clear Air Turbulence from the Ends of Invention ever since I first read the novel 26 years ago.
Making a sensible movie of Use of Weapons would be a challenge I suspect, but could be rather good as it does have some rather cinematic scenes (e.g. Zakalwe's visit to the Ethnarch Kerian).
I hope Hollywood doesn't touch these, and some independent or little-known director actually makes them into good sci-fi. Hollywood has a way of making sci-fi laughably stupid.
Actually, I'd like to do this. Maybe I'll start by making trailers.
I tend to think about the "nice" parts of Consider Phlebas (i.e. the Culture refererer Fal 'Ngeestra) during the verses (When she was just a girl/She expected the world/But it flew away from her reach so/She ran away in her sleep) and then the extremely violent parts (Orbital's been blasted, escapes from the GSV) during the chorus (Para-para-paradise....). :-)
Only in the past year did I pick up the Culture series, and I agree entirely; "The Player of Games" is one of my all-time favourite books. Absolutely brilliant.
It's such a shame that we lost him last year while still quite young - https://en.wikipedia.org/wiki/Iain_Banks