I've learned to like my backpack straps on my Soundwear Performer quite a bit, but I don't use them most of the time. The bass is too clumsy to carry indoors or up stairwells, and it's much easier getting a bass through a door sideways.
I like the straps however when I have to schlep the bass long distances over several blocks. It's more comfortable and easier to maneuver than having a wheel and I think I'm actually moving faster with the bass on my back then rolling it. It's especially great on the steep hills we have here in San Francisco - much more than any wheel. It feels more secure on my back to be honest. I bend forward a little so my legs don't hit the bass so much while walking. When I'm about to go indoors, I take the bass of my back and use the shoulder strap.
And then there's the added thing of not having to carry a wheel since the straps are permanently attached to the case and can be stowed away in it's own little compartment. Also when it's on my back I have two free hands to carry gear as opposed to having to hold on to the bass with one hand when using a wheel.
The wheel on the other hand is better at having to maneuver tight areas. I just whatever accessory that makes sense for where I'm going, but I never use the backpack straps and wheel in the same trip.