+1 on the JR Manta XL. I use one on short rides and it's great. I like it because it's wide rather than tall, so it doesn't obstruct your view of your gauge cluster.
That being said, if I'm going on a longer ride, especially in the twisties where I'll need to get down on the tank, I use a Fieldsheer Expander tail bag. I LOVE that thing. It's got 4 bungies made into the bottom of it and I just strap it down and under the tail and hook them together. As the name says, it's expandable, so you can fit a TON of stuff in there. I've even thought about picking up a set of the matching saddlebags that connect to it.
If I'm looking at a long trip or going somewhere that I need to lock up my gear, I'll pop my Givi top case on the back. But I'd rather use the tail bag on most trips.