I fall into the category of making that mistake once, but never again. My current 09 135i does not have sport seats, and while I don't find them terrible (maybe because I'm not used to sport seats), after sitting in another BMW's sport seats, it made me realize what I was missing.
If you're not sold on the seats themselves, then consider the resale value. My car was recently appraised at $25,500, but I was told that if I'd had sport package, the offer would have been ~$28,000. A $2500 bump in value from a $1300 package? Sign me up.