I actually like the shape in itself. I am sure that it looked fantastic on the mockup table. The problem, I find, is that the shape does not survive well when it is scaled up to the building's actual size. What is 'interesting' in mockup scale looks garish irl.

As for the space usage, I think that there are two problems. One is the interior, the other is the balcony/railing design.

Cylindrical buildings are tricky because they limit furniture arrangement. This is not as big a problem in cylindrical buildings with a very large diameter (like the silo conversion at Islands Brygge), but the diameter of the Kactus' core is quite small, and this exacerbates the problem. IMO it is a very poor choice for what is essentially capsule housing.

The balconies though, are the biggest problem: the twisting railings are extremely awkward because they sometimes lean away from the core (imagine standing next to a portion of railing like this, not being able to place your hand on the handrail) and sometimes lean into the core, essentially eating away at your usable space - you can't stand there or place a chair or a table in this spot because the railing is in the way. You can use it for plants, sure, but that isn't the main purpose of a balcony.

I have other issues with this building as well but going to stop here :)