I never tried it but was reading the SD thread and the biggest benefit seems to be you get to select a monthly free reward like $10 in free food or household supplies.
I already spent the $50 they had a 25% off any toy and cancelled the renewal.
Extra 30 days for returns, free 2-day shipping (with some next day), free same-day delivery on $35+ orders, monthly freebies (mine was $10 off store branded food), and premium access to sales.
Seems worth the $0 price to me. Not sure it is worth anything more than that.
First round that I'm sitting out on too. More to do with a general malaise I've been feeling toward LEGO lately, added to the fact that I've been shifting my spending more into Pokemon cards (when I can find them) these days.