I have the mono keepall 55 with strap...I've traveled to Europe and back for a week each time and it holds everything. The strap comes in handy as the bag can get heavy! But CenterStageBLN is right, on certain airlines, a fully-packed 55 may cause some trouble, especiallly if you're on a smaller plane (that happened once when I was en route to Las Vegas and took those smaller regional planes).
Any keepall with a strap will be fabulous for traveling! Would definitely suggest spending more $$$ for the strap. Makes running around a lot easier!