You will have to check airline requirements for each of your flights. Each airline is different esp. the budget airlines in Europe like Ryanair and Esayjet to name a few. Your bag size listed is 28 inches long x 16 inches wide x 6 inches deep. May be too long!
Also the reviews on the overstock web site said that two people had problems with the stitching at the top of the bag. 2 out of 4 people having problems makes me think that you get what you pay for. Just don’t overstuff and you should be ok.
WFS stands for World Famous Sports with same name for web site. It took too long to download. I wanted to see if they gave lifetime warranties like Eagle Creek, Jansport and Sierra Design.

When using a backpack with a detachable day pack, I strongly suggest that you never wear the day pack attached on the back. Reasons: You are a bull in a china shop and you will knock things over as well as hit people in the face on subways and trains. Not a good way to meet the locals. The day pack should be carried on your front chest so that you can keep an eye on your valuables. If it is on your back, you won’t see thieves unzipping your bag or cutting it with a knife.

to continue on Packer’s point about not wearing the day pack on the back, wearing it on the front will help balance your load as well and will save your back a lot by the end of your trip.