Zulily can be hit or miss. After my first order took a month to arrive, I did some research and found that they do not have a warehouse where inventory is kept on hand. Once a sale finishes, they submit the order to the supplier for that sale. Those items have to ship to Zulily before Zulily can ship them to the customer. If you know that going in, it's much less nerve-wracking to have to wait.
As for quality, I've found it best to stick to brands I know. The no-name clothing can be really cheaply made. However, there are great deals to be found on a lot of brands I like - Under Armour, Bogs, Aventura, Asic, Silver jeans, Miss Me, etc. If you know what size you need (returns typically aren't accepted), there are some good finds.