I bought this bag based on features along with the recommendations here, and after using it on one trip last week I'm glad I did!

I had been looking for something to use mainly on domestic flights, particularly to conventions and similar events out of state.  I really needed something that didn't have to be crammed into overhead compartments (the main bag goes in easily!).  I also have been trying to speed up and simplify my travel experiences, and a flexible package like this is in near-perfect alignment with that goal.  I say NEAR perfect because this one almost gets there.

Others have pointed out what they like and dislike so I won't repeat their reviews, but offer details on what kept this rating from being 5 stars for me.

First, the lack of internal compartments in the main bag.  My best luggage tends to have two pockets that either have elastic trim or flaps that close.  These are very handy for separating things, and quickly pulling out the customary quart ziploc bag full of liquid items when needed (I prefer the ziploc not be on or in the other contents due to previous spills).

Second, while I love the fact that the daypack connects to the main bag for toting, and it unzips easily, zipping it back on can be a pain.  I wish the zipper seam had been extended a bit more.  NOTE: later the zipper snagged and broke.  See update below.

Finally, there are little nagging things.  The daypack straps need velcro or something added to tuck the dangling ends away.  Splitting the daypack's main compartment in two would also be helpful for separating my netbook from other materials.

Update:

I had to bump this down a star after my third use.  The zipper used for attaching the daypack to the main unit broke loose, and now I can no longer join the two.  That feature was one of the main reasons I bought this.  The zipper appears to be a little too cheap for this purpose.

It's been a little over a year and so far I don't see that I have any options to get this fixed...