biggest common pros: hauling stuff, fitting odd size items, you snowboard not ski, you like being able to pull out luggage from either direction, or the best one - "just in case you need to sleep stretched out!"
(unless you are a giant, get a mummy bag, nice and padded. i swear it works)
*i've taken long drives in both my 97 M3 w/ fold down seats and 01 M5 where my girlfriend and I were both happy because:
a) she could turn around, reach back and get some bizarre little 'must have now' item which of course she packed in her luggage
b) i could continue to cater to my vice of constantly trying to set a new personal land speed record between San Francisco and Los Angeles
c) if you have a limber passenger, it IS possible to change your CD changer while still on the go and not stopping
biggest common con: not proven, but implicated in a number of noise complaints dealing with either tha actual back seats themselves, the parcel shelf, or general "back there" area.
mitigating the noise issue however is that at least for me personally and a good number of people who have posted on this board, i was able to eventually isolate and completely get rid of all noise issues. i'll never be sure if the noises were because of fold down seats - my M3, 540, and M5 all have fold downs. i've never had a BMW without fold down seats so i've no idea if i would or wouldnt have been fixing cabin squeaks, noises, etc, anyways. but for each vehicle, it took anywhere from 2-3 months after first getting them to zero out all the noise complaints i had, many with the help of the info on this board.
now if only i could easily find a fix for my current post-Dinan rattles