Instead of a backpack, I use a Bell tank bag that Wal-Mart used to sell. I carry it EVERYWHERE (srsly; it's my man-purse), so it's getting pretty darned ratty.
Now, I DO also own a backpack, but it's not motorcycle-specific. I got one of those w/ an optional hydration bladder (both also from Wal-Mart), but I've only used it a few times (cross-country move; a couple trips to the grocery store when I knew I couldn't load up and strap on w/ the tanks bag alone; once to carry my rain gear to work for the ride home later; etc). I've commuted on my Ninjette daily since getting it in Summer of '08 and
rarely need the backpack.