Whilst their products are really good, their business practices are appalling. They'll not refund mistaken purchases nor will they help you solve billing issues. Their customer service is simply non-existent.
On the positive side, they have really good mock-ups for t-shirts and other e-commerce products as well as great promo videos and logos included in the $14.95 monthly plan. DON'T purchase the annual plan (at $99.95) as they might start restricting access to some features further down the track.
They are pricey compared to other alternatives however you'll see a lot of Youtubers promoting them because they're affiliate marketers.