Shutterfly sends out great coupons for free photobooks and other products all the time so it doesn't hurt signing up for a free account anyway. But I have used them and they are rather good, miles above snapfish for customer service, coupons, and product. I'd say give it a shot, I wasn't disappointed with my books & products, but then again mine were all free so my expectations were low but free or not they look great.
you can actually read reviews on my blog about their free products if you'd like also to see what kind of things you can get for free, my mother and I are coupon queens (long before that dumb show )
I actually found Costco to be cheaper than Shutterfly, even with their discounts upon discounts. I made a 5x7 book recently at Costco and it turned out beautifully. I have done a couple of hard cover books at Shutterfly (also free) but was not terribly impressed. But it was free, so I just gave them to the kids.