I went to the Kings Cross stuff this evening.
Good: the underwater thing and the music/light grid installation at St Martins. They are quite old school rave, the first with blue lasers and dry ice and the second being a bit like a Winamp plugin transferred to 3D, but they're engaging. Also the giant anglepoise lamps are not in themselves incredible but it's funny how people interact with them.
Meh: the field of reflector flowers isn't as good as it could have been. There aren't enough of them, they're in rectangles so nothing feels very organic, and they're actually not very reflective. There was a wall of glowing lights nearer the station in the concourse by the Google offices that was also really forgettable. Nothing is very far away from anything else though so it's not like you waste your time checking things out.
Noted: it seems to have way more people trying to sell you pulled pork and craft beer nowadays, but that area has been yuppie commercial central for years now so it's not surprising that vendors have caught up.
They've learnt from the last one that putting things at ground level simply doesn't work because the crowds mean nobody can see them, which is good. Everything is up in the air or so huge you can always see. Not sure how this will work with the phonebooth fishtank though.