Whilst not suitable for a full RTW adventure, Ventura luggage is excellent for use on trips around Europe.
I have used mine for the past eight odd years on the back of my Honda Blackbird, with no problems. As the bag sits on a rigid frame and, in turn, on the rear seat it does not interfere the handling or the width of the bike. Speeds of 130 mph + with no head shakes or weaving. Having it easy to lift on and off is a real treat. With two bags zipped together you can up the capacity to about 70 litres but have never found the need.