Most of the reviews here are correct, no SEO, very basic templates, bad back end, etc, which is true.
Prostore's one saving grace is that it can sync with eBay, so if you're using a desktop solution or one of eBay's selling tools, theoretically, you can dynamically sync your ebay listings with your ProStores web front, keeping inventory levels matched. Unfortunately, owing to persistent bugs in the system not all the listings get imported and there's no way to tell which get imported and which don't from the back office. The support staff are aware of the problem and recommend resisting the item on ebay to get it to sync, which costs money.
If you're in between service providers or you normal ecommerce platform is down, prostores is cheap and quick to set up so it might work as a stop gap measure for a day or two, but I wouldn't recommend it for anything other than an emergency.
Further to my earlier post there has been a new development. I have had several orders simply go missing, the order was placed, a confirmation of the order has been emailed to me and the buyer, payment has been made correctly, but any and all proof of the order has simply vanish. I've also had an order placed over a year ago that was paid & shipped suddenly appear in the 'Ready to ship' section, it wasn't a re-order or a duplicate, but the actual order dated over a year ago. Finally I've had a couple of orders be paid for, but not moved from the 'pending' section to the 'ready to ship' section, leading to massive problems.
This has happened over the course of the last few weeks. Quite simply ProStores is not fit for purpose and will do more damage to your business than good.