I normally ignore emails that more or less explicitly ask me to advertise sites on my blog, but decided to make an exception for this one. It seems a hand-made project run by a small number of anonymous postdocs who want to help their fellows find good supervisors. And it's a community that I care much about.While I appreciate the initiative, I have to admit being generally unenthusiastic about anonymous ratings on point scales. Having had the pleasure of reading though an estimated several thousand of recommendation letters, I have found that an assessment of skills is only useful if you know the person it comes from.
Much of this is cultural. A letter from a Russian prof that says this student isn't entirely bad at math might mean the student is up next for the Fields Medal. On the other hand, letters from North Americans tend to exclusively contain positive statements, and the way to read them is to search for qualities that were not listed.
But leaving aside the cultural stereotypes, more important are personal differences in the way people express themselves and use point scales, even if they are given a description for each rating (and that is missing on the website). We occasionally used 5 point rating scales in committees. You then notice quickly that some people tend to clump everyone in the middle-range, while others are more comfortable using the high and low scores. Then again others either give a high rating or refuse to have any opinion. To get a meaningful aggregate, you can't just take an average, you need to know roughly how each committee member uses the scale. (Which will require endless hours of butt-flattening meetings. Trust me, I'd be happy being done with clicking on a star scale.)
You could object that any type of online rating suffers from these problems and yet they seem to serve some purpose. That's right of course, so this isn't to say they're entirely useless. Thus I am sharing this link thinking it's better than nothing. And at the very least you can have some fun browsing through the list to see who got the lowest marks ;)