There a couple issues with both the approach and the implementation. Using a slider element is not a bad idea by itself, as a matter of fact it's just an implementation of the Semantic differential test.
Now, take a look to implementation issues:
here you can see that by simply resizing window, I lost focus of the options and have no idea what I'm selecting.
Let's move further:
here, I selected one of your options, which colors the text (nice detail), but everything is overlapped and not only I lost focus, now I see there's even more space on top, even though I selected the best option.
What to do?
Well, first of all, you should have only 5 spots (or 7 with 2 in between selections), and the "in between" spots should be in positions 2 and 4. Having an "over the top" option makes no much sense, and not only adds a tremendous degree of subjectivity, but quite some friction. As an user, I have no idea if this is a developer mistake, if I'm supposed to choose blank choices or what.
the "connection" path: right now, you have a connecting path between choices, which adds a sense of connectivity to all options. So, right now it seems to me you're communicating that Mike the new hire is bad, so so and great at the same time. Maybe he was bad and now is great? Maybe he was great and now he is bad? Or even worse than bad since you have an option below that? If you ask me, I'd get rid of the connecting path and simply leave the options. Think
radio input elements, you can only select one, not all of them!
- the visuals: while you did a great job in implementing this request, I'd go with something closer to radio buttons. Something like this (not necessarily, just as an example):
Of course, if you want to keep on a more skeuomorphic path, you can do it, as long as the options are easily identifiable. Here you have some options you can use for ideas:
orientation: since you'll have a lot of issue with resizing as shown above, what about placing this on top and changing it to horizontal view that can easily be resized in responsive mode?
And finally, I'd add some instructions, whether you use just a simple block of text, coachmarks or any other kind of guidance, it will improve the usability of your implementation, leaving no room for doubts