I have the older Space Explorer. The ergonomics work extremely well for me. The button placement takes a little time to get used to. The ALT, SHIFT and CTRL buttons are huge bonuses. It is very similar to the Space Mouse Pro.
I've also used the Space Navigator. It does allow me better control over my models and quicker manipulation. I don't like the ergonomics. It always makes my hand feel cramped, kind of like using a really small travel mouse. The fact that it is easy to pick up, really annoys me.