I don't think being local is the total point except in the case of short weekend trips when of course, you will be traveling lightly so can easily do metro. I've used airport shuttles in my home location due to cost, and I'm not the only one. And I do know some Parisians use them, also.

However, I booked one once and it didn't show up. So not only did I have to have a backup plan (which I did), but I wasted maybe an hour of my time waiting for them, making phone calls to their office, etc. That was Bluvan. It was airport into the city on arrival, so I didn't have to be somewhere at a certain point. Given that experience, I would be leery to do it again, unless booked through my hotel who has a relationship with a company and knows they always show up. The hotels I usually stay in do not, however, have any relationships with the group shuttles, only private car services (which are too expensive for one person). I prefer public transportation myself, or Air France bus, but otherwise, I'd take a taxi definitely for the trip to the airport, as it is too risky if you miss your plane.