I've been using Kriega US-Drypack soft bags on my Zero SR/S, instead of the Shad hard bags that can be bought with the bike. I can't claim thousands of miles of experience using them yet, but so far the quality of the Kriega bags seems right up there, justifying their high price.
I'm using a combination of US-10 and US-20 Drypacks, with my charging cable, lock and waterproofs stored in the US-10, while my clothes and travel stuff is in the US-20. I've chosen to use two bags so I don't have to worry about getting my stuff wet or dirty after charging in the rain. I could wipe down and clean my cable after charging, or have another inner bag in which to put the wet charging cable, but in both cases I'd be opening the bag with my clothes in the rain to stow the cable.
The Kriega US-Drypack system is modular so that bags can be attached to each other. I'm not entirely sure why the US-20 has loops on its top, as the top bag should be secured to the loops on the bottom of the bottom bag.
The other small benefit of two bags is that I can leave the US-10 packed and ready to go, and if I'm heading out for a day ride I just strap that on by itself.
The downside to using these bags is that the Zero top rack is not the best for installing the web loops, as you can see below. The rack is just about large enough to support a US-20, but because of the position and orientation of the lashing slots I've ended up with this concoction for the web loops with the bags half on the rack and half on the rear seat. It must be possible to do a better job than this.
