They are Huge! You can fill it with everything you will ever need to travel with for years but they are heavy and cumbersome. I remember my hiking teacher at high school telling us that you should only carry 1/3rd of you body weight on your back because any more than that can cause serious damage to your back, knees and shoulders. Most airlines have a 20kg luggage maximum so you can't take the kitchen sink. I am not a fan of these for basic travel. If you are not having to take a tent and sleeping bag I would leave it at home.
Wheelie Bags and Cases
Very easy for packing in and is a popular option for traveling if you can keep it in one place. I have traveled with one before and found it very good but was a bitch to walk on uneven terrain with and get in and out of train carriages. The rigid structure of the case keeps everything inside in perfect condition even when the outside gets a little battered. More suited for city trips than long range traveling.
The best of both worlds, you can wheel it in the city but you can carry it on your back for fast access. I am a little in love with them for there versatility but they are a little smaller that the big hiking packs and can be a little uncomfortable to carry over long distances because of the steel uprights for the handle and wheels. I am seriously thinking about getting one of these for my trip.