Preview night is crowded - you might see a queue around the block waiting to get in! But I went to preview night last year, and at one booth, found some coordinated fabrics they were selling at a deep discount - when I came back a later day, those deep discount fabrics were gone - so you MIGHT find some great deals on preview night. ... Another thing I like about preview night (since I usually just go preview night and one additional day) if you see something you REALLY like, but it's pricey, you can go home/hotel and research it online to see if it's really the best price at the quilt show, before you take the plunge and buy it.
BTW, they often have great deals on sewing machines. etc, and they do sell the basically-new machines they've been using in their classes during the show, at a discount - in case you're looking! All the top name sewing machine mfg are there with LOTS of machines. I think it's an exciting show!