I've completed 2 orders now through 599 and really love it.
I hate spending a lot on clothes (especially over the past several years where my weight kept going drastically down and better yet now that I've gained quite a bit back). I've never been a fan of clothes shopping: finding stuff you like only to find the one size you need is out, trying things on one after another, the constant undressing and re-dressing because you need to go swap sizes or don't like how something looked so you need to find something completely new... Gah. Just not my idea of a good day.
Being able to browse the site casually and think on things for a few days (granted, running the risk they get sold out in that time) is a dream. Their returns policy is super easy and really customer-friendly.
I've had to return just 3 items out of the I-don't-know-how-many items I've purchased and their process was super easy (plus, I like the options you have for a refund, credit, etc; I've gone with credit because I figure I'm going to get more anyway). They have a wide variety (at least for women, would love to see more selection for men!) and great prices.
I've seen people complain that not all their items are $5.99, but being able to get some of these shoes, jeans, and/or dresses at $9.99 (I think that's the highest price of anything there) is still a freaking steal.
Anytime I've ordered something that turned out to be out of stock, they've been quick to email me with different options (if I wanted a chance to get a different item instead or if I'd rather get the rest of the order ASAP with a refund or credit, etc etc).
I've had a great experience with this store, even with the little disappointments/inconveniences like an out of stock item or returns.
I actually get excited now about clothes. It's a lot more enjoyable trying on my orders (like the spoils of war!) versus trying to hunt things down in-store(s) and combine the "do I like this?" search with the "will it fit?" drudgery in the same go...
Having the size chart is nice, but having the measurements for the models pictured and stuff *really* helps with getting a pretty close fit off the bat (we all know how sizes can be inconsistent through different brands and styles).
I can see being a long-term customer, easily.