What about the Google Nexus line? They're neither very expensive devices nor crappy and underpowered. The entire Nexus line has proven that there's a demand for decent hardware at a non top-end price out there. It's especially important when it comes to being "allowed" to flash whatever ROM you like or unlock your bootloader, plus the pure AOSP experience means that updates aren't delayed by vendors or carriers.
For sure, Samsung are providing top-end Android devices, but the Nexus 5 is competitive with all the Galaxy S type handsets currently out there, plus the iPhone 5/5S too, and at a fraction of the price.
I agree that the Nexus line serves this niche. However, I do think there's still space for one more entrant in this market. Plus, the fact that Google is increasingly closing off bits and pieces of core Android behind Google Play services means that there's a sub-niche out there for a phone that's truly open. I'm not sure if it'll be enough to carry this device to profitability, but I'm certainly hoping that it will be. I was planning on upgrading my Nexus 4 to a Nexus 5 this year, but after looking at the specs and software on the OnePlus One, I'm probably going to hold out for one of these. The site says they're supposed to come out in Q2, so I shouldn't even have to hold out that long.
For sure, Samsung are providing top-end Android devices, but the Nexus 5 is competitive with all the Galaxy S type handsets currently out there, plus the iPhone 5/5S too, and at a fraction of the price.