River Road makes a Tank Bag System that looks to be as close to ideal as I have seen.  There are three well-designed pieces to this system.  First is the magnetic base that has a built in waterproof map pouch for use independent of the bag sections.  Second is a large bag that zips onto the magnetic base and contains a set of backpack straps for off bike transportation.  The last piece is a smaller bag that can zip on top of the large bag or directly onto the magnetic base. 

I like how flexible this bag seems to be.  With all of the different combinations it has the right size setup for almost any need (within reason).  I was not able to spend as much time in person with this bag as I would have liked but it seemed to have a good inside layout.  Places for small documents, pens, pencils and other office type tools.  And the big thing–it is big enough for my laptop.  Something bag manufactures often forget when designing bags.

Reviews of this bag seemed to be positive.  Most people said this bag was a lot of bag for $150.  Build quality seems to be adequate, holds up well and the magnetic base was plenty strong to hold the large size bag.  Reviewers commented that the zippers are larger than zippers on most tank bags making it easer to reconfigure and detach at will.

The down side of this bag is its size when fully stuffed.  I read a review of this bag by a 6’ 1” tall person who was having a hard time seeing over the bag.  He had it mounted on the tank of his SV650.  I’m 5’ 8” so I don’t think I would get much use out of this bag fully stuffed on my SV650. 

All that being said I think this is probably the most useful tank bag for general commuting and overnight road trips I’ve seen.