Re: Tank Bag/Seat Pak
Had the 15L, replaced it with a 20L. IMO the 20L is a more practical size. The 15 was a bit small, although if you just want to carry a few odds & ends on a day ride it is plenty big. I had used the 15 for touring and got in a spare shield, wee willy, a digital and 35mm camera, and a few other bits. I like to ride with it unexpanded, so I could have gotten in more.
Be sure you get the new style with the additional side flaps with magnets. The old style was a bit tippy and unstable in strong crosswinds. The new one is rock solid.
No longer hanging out here, sorry.