I've got one that I've been using for about 3 years now (Awesome fun fact- the Ninja 250 and the SV650 use the same size bag, so I didn't have to buy a new one when I got a new street bike
). I use it to transport my purse largely, but even with my decent sized clutch purse in there I can still fit any manner of takeout box or a couple quarts of oil or a crumpled up jacket or any other smallish thing I've ever needed to transport. It actually fits a surprising amount of stuff. It looks pretty low profile zipped up, and while they don't recommend it, I've also carried a passenger on it no problem. I've ridden with it through canyons and it doesn't flop around at all when it's expanded and I never need to worry about bungee cords scratching my paint or what have you.
My only negatives are that, after 3 years of california sunshine, the black of the cloth that wraps around the seat is a little sunbleached, and the seat takes some effort to get back on after putting the bag on it. So basically, I have no real complaints
All in all, really great and extremely useful product. Trumps wearing a backpack any day
Oh, and this picture is from a REALLY long time ago, so pay no attention to the laughable technique and still totally stock bike, but here's what it looks like on and half full:
