I went to the 4pm session yesterday and listened to Cosmic Music and Attica Blues. A couple of shaky cameraphone pics below.
I found it a really interesting experience. The rear channels were less pronounced than I expected and were generally drums/percussion - I guess the idea being that you're on stage with the drums behind you? - though the odd other instrument popped up at times.
Of the two LPs I thought Cosmic Music worked best in quad. It was great to be right in the thick of it. It was a bit disconcerting at first how the instrument positions changed between tracks but it kind of added to it's charm - the record has kind of a quirky almost DIY feel to it anyway.
I found Attica Blues a bit of a mixed bag. Some of it sounded gorgeous in quad, some of it sounded a bit congested and in your face. I suspect this is a product of the production as much as anything else. It's not a record I know very well so I need to track down a stereo copy and compare.
The quad arrays of Quad gave out a surprising amount of low end. It looked like they were powered by 303s on this occasion. The organiser Elan is obviously an ESL fan - mentioned at one point that he had 22 of them!
I won't be rushing out to buy a quad decoder but if I had a large listening room that could accommodate them it would be an excellent excuse to have a second pair of speakers - great fun.