I was hoping someone had come along who had bought one! I've been thinking about these too, to give Ds a bit more freedom especially while we're camping and for peace of mind in busy places when he darts off to look at something.
Last year I thought perhaps I'd be getting him a present for my convenience, rather than something he really wanted, but he was really excited when we spoke about it. He didn't end up getting one last year but it's under consideration again. It would mean I could bump some toy and gift requests to GPs etc and hopefully cut down on the tidal wave of bits and pieces that threatens to engulf us. Please no random tiny sets of Meccano type stuff that is not Meccano!