I haven't used any of these bags, so I can't say anything good, or bad, about them. What I can say is I have a Jansport backpack with the removable day pack (paid $75 on sale, killer price). I love it! The versitility is awsome. Only thing I don't like is it sticks out a bit much when the 2 bags are zipped together. You really have to be aware of the room you are taking up. It is easy to knock something (or someone) over. Look on eBay for backpacks, lots of good deals, and a large selection.
It's not how you play the game that counts, it's how the game plays you.