Hi John
I'm not a CAD power user, but the guys at work are and use the hell out of Inventor, which is similar in terms of navigation to Fusion. They all use 3Dconnexions mousy thingies and they all love them. They've had these and similar ones for years, and I think they would leave if I tried to take them away now.
But I am an occasional user of Inventor and use Fusion mainly for the CAM bit at home, and I get along no problem without one, which is probably not least because my brain needs the delays to work out what the next steps are. And I agree with Muzzer, there is a long learning (or brain programming) phase – they are easy to operate, but not easy to get them to do what you want.
Anyway, for my money, if you are an occasional user, don't bother. If you competent with Fusion and use it a lot, then yes, they are great.
Cheers – Will