Hi Liz, yes I have driven over that one in Swindon, there is also a similar 'magic roundabout' in Hemel Hempstead.

I have been over the roundabout now without lights this morning, it was a lot smoother (but not the busy time). I did have to wait a while to get out of one the smaller side roads onto the roundabout, probably no longer than one would have to wait for the lights, but I imagine at busy time getting out against the mass flow of traffic isn't easy.

I think popular opinion is forming to suggest that the lights should go on at rush hour only.