Well, I'll answer my question, as I finally got round to using them this weekend.
They're great! While I'm a huge fan of the metro and RER, there are some journeys that are a little hassle, particularly with luggage, and particularly when they're out of the city centre, and this nippy little electric cars do a great job of getting you around (for €9 per 30 minutes). And, well, a drive through the Louvre, along the river and then up the Champs-Élysées is nicer than RER A, even if I hadn't anticipated that the Étoile tunnel was shut and I'd have to negotiate one of Paris's more 'fun' junctions in a small, unfamiliar vehicle... But then up and under La Défense, doing pretty well on the motorway as well.
Very handy and rather fun, albeit in varying states of cleanliness... The collection and drop off procedures are pretty straightforward, but the one hassle for casual travellers is that you can only get a card to get access to the vehicles if you go to a subscription kiosk, of which there are far fewer than there are car stations. And you need to do this for each 24-hour period.