Jolla is a company that was formed by former Nokia employees in Finland, soon after Nokia announced it's going to kill its Meego operating system, and go with Windows Phone 7 instead. What came out of that is Jolla's Sailfish OS, that is being used on the phone being launched now.
Sailfish OS is based on Meego, but it can run Android apps, too, much like BB10. But just like BB10, it doesn't actually have access to the Play Store, and it needs its own store for Android applications. Jolla has partnered with Yandex (the Russian search engine company), to get its Android app store with 85,000 apps.
People who loved Meego, who weren't that many to begin with, because Nokia never really gave Meego, or Maemo for that matter, a real chance, are going to like this OS for its flexibility. In fact, many of them are probably in that first batch of 50,000 pre-orders that were already made.
The Jolla team does seem to have retained some design chops from Nokia, as the phone itself doesn't look too bad, although I'm not overly impressed with the specs for the price. It comes with a dual core 1.4 Ghz processor, a 4.5" display with a 540×960 resolution, an 8MP rear camera, 2MP front-camera,1 GB of RAM, and 16 GB for storage – all for 400 euro. That seems a little overpriced considering you can get a Moto G that costs a little more than half of that price with about the same specs (different compromises).
There could be a whole number of reasons why the Jolla phone is this expensive. The company couldn't order the components at a good price, since they don't know if they are going to sell millions of them. They may have ordered them a while ago, when the components were more high-end, but they were too slow to take it to market, so now they are stuck with components that are more obsolete but still cost them a lot.
Either way, I don't see it as a great value phone, especially when you consider it doesn't come with the Play Store. That doesn't mean it couldn't be "successful" enough to keep the company going, though. There are many less known local Android OEMs who keep churning Android devices year after year, so if the management is competent and tries to offer higher value in future generations, I can see them hanging around.
As for becoming a huge success, as the 3rd or even 4th mobile platform, I'm a lot more skeptical about that. To do that, you really need a multi-OEM partnership, so that OS is on many smartphones from other companies, too. In that case something like Ubuntu Touch or Firefox OS, which are targeted at other OEMs too, will probably have more success in terms of global market share.
The Jolla phone is now available for sale in Europe, and you can buy it straight from their website for €399.00.