Hi! I personally have been ordering from Zulily for quite some time now, and I absolutely love it. I often see bad reviews based on how long shipping takes sometimes. In my experience, MOST of the time the shipping is very accurate to what they post on the item. Simply look at that, and if you don't want to wait that amount of time, just don't buy it. Zulily sends their orders to the company who made the item, who then ships to Zulily, and only then does your item get shipped to you. So Zulily is dependent on all these companies as well for that. If you are okay with this and the shipping time on your item, then you're good to go!
I'll address quality of items as well. Again, Zulily sells batches of items from different companies. Some are big, well known brands, others not so much. Most are of excellent quality from my experience, but sometimes you just won't like it. It'll not fit right, be poor quality, etc. You'll see this in any store, but here we are talking about a website where you cannot physically preview it before buying. It's just a risk you take. In many many items, I've only been displeased by 4-5 of them. In those cases, Zulily customer service was superb and worked to get me fully compensated, no questions asked.
I have really found some truly unique things on zulily from small or foreign brands that I adore. But keep in mind you're taking a chance, and chances don't always pay off. But zulily WILL work to compensate you, so there's that.