Shop around and sit in each year model on the lots. Make note of things like Dash type, LED read out, rims, tires, arm rests, engine size, cruise control, navigation, etc. etc. NOT only is there a diff between 06/07 and the 08 model, there are TRIM Levels as well, so not all M5 will have side skirts, alloy rims, etc.
Good luck, post pictures of what you get so we can point out all the flaws, NO just kidding, and good rule of thumb, cars that are newer are supposed to be designed better/greater, not the other way around, so you'd think an 08 is better quality than a 06 or 07 (laugh)
Peace!(rei)
P.S. Sad thing is 124 other people viewed this post and had nothing to say? :0