I really like it.
Early in the game, I don't need a lot of boxes, and it's actually much easier to get around in them if there aren't a lot. Later, when I need more boxes, I can open more of them up. Until I need them though, they'd just be in the way, so it's nice to simply not have them there.
And personally, I like the way that they're opened, since it's entirely in my control. If it was based on something like the number of pokedex entries, then they'd just open whenever they opened, regardless of when I wanted them open. But since all it takes is putting one pokemon in each of the existing boxes, I can do it, or not do it, as and when I choose.
The only sort of problem with it is that it wasn't explained in BW. I assumed that more boxes were going to open sometime though, since there's just no way that GF would limit us to eight boxes. I didn't know when or how they opened (even after they did in my game) until I happened to read about it online. In BW2 though, it's explained by NPCs, and IIRC, more than once.