Storenvy is hugely popular in the US, but it isn't Etsy. There are a lot of mass produced items on there, but Storenvy has never been advertised as a handmade market, it's more a place for designers, and if a designer has their products mass produced, it counts.
I sell with them, but I have a hard time actually SELLING. I think that's BECAUSE my stuff is handmade and so not as flawless as plastic items.
As Amanda has said, it's totally free. They make their money through a t-shirt printing company - originally it was all one website, but both the selling aspect and the t-shirt printing part totally exploded so they seperated the sites. Storenvy only make money from sellers if sellers pay a monthly fee which gives them access to offering coupon codes that give buy one get one free offers, or fixed discounts (ie $10). Percentage discounts are free to use, though. I've been on there for about 6 months and made 3 sales, so I don't bother that much, but since I don't have to renew any listings either I can just leave it be. As long as I remember to remove items that are no longer available, there's no problem, and occasionally a sale will happen out of nowhere.
But Storenvy don't do any advertising for you like Etsy does. Etsy looks after us, and that's where the fees come in. Storenvy don't. All they give you is, effectively, a free page and a free shopping cart and checkout. You have total control over your shop's HTML though, so you can make it totally your own, and you can add your own URL too.
If you have the time to manage them both then go for it, there's a feature that allows you to copy Etsy listings onto Storenvy too, the first 20 are free but then you have to pay about $20 one-off for the rest of the app that will allow you to copy the rest.
It's not great if you're just starting out, and now that Etsy allows Direct Checkout, Storenvy is a little more redundant since you no longer have to redirect people there who don't want to pay by paypal.
