I'm rather obsessed with carry on luggage these days as I'm too cheap to willingly pay to check a larger bag. Accordingly, we have four different carry on bags - wheeled only, backpack only, and combos that do both. I reserve the wheeled only bags for short domestic trips. We recently did a three week France/Italy with only backpacks (eBags TLS Mother Lode Weekender Convertible) and loved them. I really like their organizer pocket for pens, receipts, change, etc., that seem to amass during a trip. However, I also like having the option of wheels - like when I'm sprinting between terminals in some giant continent sized airport. And the expanding feature in the eBags bag can get you into trouble. I've found it difficult to fit in into an overhead bin on domestic flights when it is really stuffed.
I recently found a great sale price for a Eagle Creek Lync System22, which is very light, but so far seems very sturdy. It has wheels and backpack straps. So far, I like it. I took it on a 3 week driving trip across Maritime Canada. The only downside is that it has sloping shoulders and has slightly less volume than the more squared off eBags bag. I didn't use the straps often, but it's nice to have them when you do - like climbing stairs at smaller hotels.
For that same Canadian trip, my wife got one of the Rick Steves Rolling Backpacks and she loved it. On sale I think she paid only $130 and it compares favorably to my $260 Eagle Creek bag. To be sure, they are different bags, but the RS bag has both wheels and backpack straps and their customer support is top notch. A seam came loose during the trip (I suspect overpacking was largely to blame) so I e-mailed the RS folks a picture while we were on the northern tip of Prince Edward Island and we had a new suitcase shipped to us by the time we got home. Even a well made bag can be stressed too much. I like the Eagle Creek bag, but in value for your money, I'm very impressed with the RS bag.
The moral of the story? Go with a quality wheeled/backpack combo and you will have the best of both worlds. That, and learn to pack lightly. As Rick says, I've not once come back from a long overseas trip wishing that I had packed more.