I love Iain Banks and have everything he's ever published on my bookshelf. Only Tove Jansson and Graham Greene have a similar status on my shelves. I enjoyed Excession but it's quite difficult to work out what's going on as most of the action/dialogue is between ships and I found that I lost track of who was who very often.
I'd suggest starting with Consider Phlebas, the Player of Games or the Use of Weapons; much easier to get into and follow, and they'll give you an awful lot of backstory which will help when you get to Excession. Think I've read the Player of Games 5 times now...