I have a Kriega US-20. Great bag. Has kept my gear dry and held up well. I find it a bit small though, so I pretty much only use it for hauling my laptop and lunch plus a few odds and ends back and forth to work.
I also have a set of Twisted Throttle Dryspec bags (found here at A Vicious Cycle):
http://aviciouscycle.ca/MainPages/searchpage.aspx?keywords=dryspec. I bought the two-bag kit before a trip last summer. It had loads of room in the big bag for all my camping gear, and I used the smaller one for clothes, etc. It fastened down securely on my SVS, and was definitely weather-tested, with everything remaining 100% dry.
Both are "on the seat" type setups so you don't have to worry about the bags scuffing your tail section or side panels. Definite thumbs-up for both Kriega and the DrySpec bags, but if you need the room for gear, I'd lean towards the DrySpec (plus the DrySpec combo kit is cheaper than a 3-bag Kriega setup). Thumbs-up to A Vicious Cycle out by Kitchener too - great service.
Edit: The DrySpec bags are quite long; longer than the passenger seat. I have a small SW-Motech rack on the back of my SVS so the overlap onto the rack wasn't an issue. Without a rack these bags are long enough that they would lay on the tail section of your VFR.