I have literally placed dozens of orders on Zulily. The main problem is that you don't always know what you're getting until it arrives at your house. If you don't like the color or the way it fits, there's not much you can do. If an item arrives damaged or not as described, Zulily customer service will promptly refund your money.
Yes-it takes a long time for them to ship the items, but they are up front about that on the website. You place the order with Zulily, they collect the orders and place one large order with the company they negotiated the discount with. The company ships to Zulily; they ship to you. That's the trade off: cheap clothes, slow shipping.
I've scored a $400 designer dress for $60 that fits beautifully and looks fantastic. I've also spent hundreds on an order for a summer wardrobe, all of which were flimsy and unflattering. The key, I've discovered, is to order a reputable brand, not a "no name" company. I don't always follow this advice. Occasionally I see an adorable top I have to have from a company I can't find any information on. Sometimes I end up regretting it, but not always. Zulily is a mixed bag. Just order with caution.