I own and use a 17" and a 15". Prior, I used a 12" PowerBook.
IMHO, the 17" is a great desktop replacement but I do not think it is a great laptop. It is portable but only to a point. I don't think trying to find a bag for it should really be an issue of consideration as there are loads of them about, though not as many as for the 15-inchers. Airplane use is indeed a nightmare for you and your neighbors though and since the power drains rather faster, I have rarely made it through a full two-hour DVD without dimming the screen to a mere shadow. I love this computer, but for the most part it stays at home or office.
The exception to that is when going onto a film set. Having a larger-sized fully portable screen is invaluable when needing to review dailies and takes on the fly - it has been my experience that only 4 people (5 max) can watch the 15" screen at the same time with visual clarity enough to critically watch color and movement, whereas 7 or 8 can be accomodated by the 17".
But the 15" is all a much superior computer when portability is really the purpose. In the original post, it was said that this is not a replacement, but an addition to other larger sized desktops. I gotta say, if this is to be used while away from the office or home or on vacation, or while travelling around the world or anything like that, go with the 15". The extra power, extra screen real estate is just overkill if you already have larger systems. Unless of course you can't do without it and have no other use to put that extra $800.