I use the Yamaha bag (from FJRgoodies) for long trips. Very large, expandable, waterproof, extremely well made. 3 years and not one problem--but it is large and may get in your way if you have a large gut. As mentioned previously, it does touch the controls slightly with risers. I get the occasional unwanted horn beep..but no biggie.
For short rides, I use nelson riggs. Also waterproof (I think!) and well made. Not as large, but I barely know it's there. Expandable by only a small amount (enough to cram a heated vest into a full bag, but that's about it).
Both are magnetic.
Yamaha was $150,nelson riggs was $50. Both serve the intended purpose very well. Also have a matching Nelson riggs tailbag I use for water and tools. This mounts on the rear seat. I use the bungie buddies to attach it.
Stay away from sliptreamer. They are thin and the threading does not hold up to use/abuse.