I prefer a rack and panniers for carrying most things. However, my road bike is not equipped that way. I have a carradice SQR slim on the road bike. SQR = seatpost quick release. The SQR mount stays on the seatpost, however, it is very lightweight and not really noticeable. The slim bag fits right underneath the saddle and is attached and unattached to the mount very easily. You can check out the pictures and description at:
http://peterwhitecycles.com/carradice.asp