I was opted out BUT when I went to double check, I was mysteriously opted back in. So, I opted out again. I only have a few items in my store (less than 40). The last week or so, I've sold quite a few. I woke up to 2 dresses sold over night. Both with Best Offer on them but they paid full price. Cool! I've tried Best Offer over the last 7 to 10 days with good success. Of course, my items are listed at $24.95 and lower. It wouldn't be good for high end (expensive) merchandise with slim profit margins.
Opt out of eBay Express and report back.