Originally Posted by Grim

MTX DXP
http://topeak.com/products/TrunkBags-MTX
Yes it has fold out panniers but since they are on the side of the bag they actually ride higher then a conventional pannier. A true 4 season bag. In the winter you have the panniers when you need to shed a couple layers. In the summer the main compartment is insulated and you can take a water bottle get it near freezing and stick it in there with you lunch to keep it reasonably cold.
My only negative comment is it tends to collect extra junk (weight).

Now let me also point out another option. I have used my MTX for 2 years and while happy with it I recently tried out a mediums size handle bar bag. The gain is I can strap my iPhone on top and use the GPS, speaker phone and Ipod (on speaker) as I roll. My camera is very handy as is other items without having to fully dismount. In fact a lot of stuff can be grabbed as I roll if I need it. I have gone to a small seat bag for my patch kit, tools etc. I keep my personal junk in the bar bag. When I need to shed a couple layers I bungee it to the rack. I have the cheap bar bag I picked up from performanc on sale fro $32. I strung a bungee under it off the bars to steady it. So far I am liking it quite a bit.