I have used the High Sierra ATQ bag traveling through Europe in the past, and I would say.... that it's the absolute largest bag I would consider traveling with again; that's to say, I got through everything just fine, but overall I would recommend a smaller bag and packing lighter.
First, it's not a carryon size, so you're going to have to check it. It's a great bag in terms of storage space, durability, etc., so that's a huge plus. However, being a rolling bag, as some have mentioned, it's at least 10 lbs. by itself with nothing in it. So when you do fill it up packing, it's going to weigh a good bit more. WIth that weight, I used it as a rolling bag and a backpack about half and half. Using it as a backpack is great when you're in cities like Venice (constantly going over bridges) or going through stretches in cities/towns which aren't exactly paved (cobblestones/Cinque Terre). Being young and in shape, carrying all that weight on my back wasn't a problem (a little work-out, but no problem)... but if you have a bad back.. and with all that weight... I wouldn't recommend it.
Anyway, I still have the bag and I like it a lot and will use it again. However, for traveling through Europe, I would recommend a smaller, lighter bag, as I have traveled smaller and lighter each time since. If you do want to carry your luggage on your back (which I have done so since and would recommend, as it's very convenient) then I would suggest a non-rolling bag, as it's just added weight you don't want to be carrying. I have been using Rick's Convertible Carryon since, and it's worked perfectly for me: it's a carryon, you can use it as a backpack, it's light (since you're packing light to fit everything in anyway), and with that, the perfect size. And if you want a daybag, just get Rick's Civita Day Pack, or for an even lighter load, Rick's Pocket Pack; both of them can fit into your bag taking up virtually no space at all.
But I hope this helps! Enjoy your travels!