Why is it not usable? Lack of apps? I am guessing the lack of keyboard integration makes it difficult to use "Gemini style" (ie. via the keyboard).
One certainly wishes for more apps, but that's not unexpected.
And it's generally buggier than Android (unsurprisingly). e.g. I occasionally find myself with no network connection, and need to reboot to get it back.
But, off the top of my head, the important niggles that I can remember:
* No real keyboard support. Or I haven't found it at any rate. So I can't even set the keyboard to the UK layout (although that is the language I selected). And most of the special keys don't work. So the keyboard is difficult to use and doesn't provide much help in doing things (one still needs to use the touch screen for most things).
* No GPS support. So navigation and maps aren't very useful.
* The side button doesn't seem to do anything. So one needs to open the cover to take a call.
* I won't swear to it, but I think it always uses both microphones when talking, which doesn't help with call quality.
* It always seems to use just the left speaker for telephone calls. So one needs to be careful which way one holds it.
* LTE does work well for me. But it seems to confuse the telephone app, so it refuses to make calls unless I change the settings to use only 3G. (One can however text and receive calls when using 4G). This is obviously impractical.
There are no doubt other things that I've forgotten or haven't discovered. The bottom line is that it is still difficult to do just about anything with it.
It does appear that it wouldn't take a lot of work to fix it, but my impression is that not much is going to be done until Sailfish 3, so we'll just have to wait for that. If someone wants to correct me, and get me a new version, I'd be happy to help iron out problems with it.