Big is a relative term. LOL.
I have one that I use mostly at home, but that I take on the road about once a month when I travel. The laptop bag I have is a generic 17" PC laptop bag, so it's a little bigger than it has to be. The bag fits under the seat on the plane, or in the over head compartment. Laptop is really too big to use on the plane, except in first class, although I have done it before, by placing it on your lap.
It is certainly bigger and heavier than the 15" when you travel. That said, I use my laptop as a replacement for a desktop, so I really wanted a big screen.
This is my 2nd 17" machine (I had a 17" PB before the MBP), and after using the 17", I didn't really feel like I wanted to go back to a smaller screen for my day-to-day work.
I looked at the HD screen option and decided against it, because everything was just to small for me - it made it difficult for me to see things. It stinks to be over 40! Until the "resolution independence" function is available in Leopard, the HD is just too HD IMHO.
I would definitely recommend that you take a look at an HD model in the store before you buy one, and see what you think...