I bought the bag on site at a tournament after some very helpful discussion with the vendor. Here's my promised review:
This bag is very versatile but it will not excell in either trolley or backpack mode.

General: I like the small compartments that leave enough space for stuff like bandages, extra tenugui, flip-flops, a bottle etc. The shinai holder is nice too (I would say 3 shinai are ok, 4 are borderline).

As a trolley, I like the weight balance and its sturdyness. It tumbles over much less when dragged than my previous bag. Minus points for the design of the handle: When pulled as a trolley, the main compartment is not sealed off completely, so rain get inside along the handle.

Backpack: Mind you, I'm using it mostly as a trolley. I only use the backpack function for e.g. changing trains and long stairs. If you want to use it mainly as a backpack, buy a backpack. The back part is not heavily cushioned or very ergonomic, so it might get uncomfortable for longer distances and it does not have a hip belt. But the backpack straps are otherwise comfortable for short distances even if it is a bit too high for a backpack (I'm a short female).

In backpack mode, you could hide the weels between the flap that covers the backpack straps in trolley mode. However, I usually leave the flap open and fix it below the rubber strap on top. The backpack straps are still usable this way. So I can always switch between backpack and trolley mode without any changes. This works fine as long as the wheels are not very wet and dirty (this way they will touch my clothes). I find this a good compromise and it is more comfortable to carry the bag on my back then to lift it one-handed.

Great bag!
I received my order today and this bag is really extraordinary! My bogu fits in perfectly, with room for Hakama and Gi as well! Really, a great purchase!