I was included in the 5% of the community that were to test out the new interface and i can honestly say that i opted out within 10 minutes of using it. It doesn't yield relevant results, it takes twice as long to load up pages and quite frankly is crap.
Take this example. I ran a search for test tubes using the new UI, about 10 results came up - the majority being CD's. With the old system, about 70 results were returned, almost all lab related. That's a bit retarded if you ask me. The main problem being that almost all the old sub-categories have been removed without any replacement, so hundreds of thousands of auctions that have been made in the last few days seem to have just vanished and it's almost impossible to find them.
I really do hope that eBay do something about it before they make it set in stone in autumn.