If you’re happy to be an old fart I’d wholeheartedly recommend them. I’m on my third after having had the last two stolen (well-locked, in broad daylight, with loads of people around). As Cheesy and Simon say above they all come with the clip now so no need to worry about spontaneous folding.
I have no doubt that an Airnimal is a better bike for riding – I came close to buying one myself – but if it’s not going to be used for long rides then for me at least the Brompton’s foldability outweighs the Airnimal’s rideability. But I know of others who swear by their Airnimal, or for that matter their Birdy, both of which are much more like proper bikes.
Just, whatever you do, don’t lock it up anywhere. I don’t even carry a lock any more when I take my Brompton to London – it’s a pain to carry it everywhere but less of a pain than walking back to the station.
And if you ever have cause to go to Paris on the Eurostar I can’t recommend taking the Brompton with you highly enough, even if they do now insist on you sticking it in a bag. It’s such a great feeling to cycle off across the city from the Gare du Nord, and few things so strongly reinforce the mad Englishman stereotype as turning up at a hotel on one!