Of all the crap that I have bought for my GS, the Sargent seat and the $40 HID headlight upgrade are the two that I am most happy with.
I bought the BMW Adventure panniers for my regular GS with the bike (the deal I was offered was too good to pass up). As far as other farkles, in the order of preference and usefulness:
- Heated grips (unless you live in FL)
- GSA winglets
- Engine guards (the aftermarket ones are little cheaper)
- On board computer (I don't have this but I do wish I had the thermometer, don't care about the other info the computer provides)
- Better windshield (GSA is OK, the GS windshield sucks)
- Beak extension (just looks naked without it)
You are better off buying everything else aftermarket.