How are they specified and how do the available materials map to the ones we're used to?
They are listed in the same way as you would find on other sites such as T50-6 or T50-2 etc. I have no way to actually test the exact materials they are made from but I did run a few tests on the ones I received such as winding a number of turns and then measuring the inductance - they were very close to the predicted values.
I have also created some bandpass filters using them and used my spectrum analyser to test them - again they were very close to my theoretical calculations.
I have made several oscillators of various designs and all of them have worked and have also been really close in frequency to my design.
For me personally this is exactly what I need - as long as I can go on toroids.info, choose the calculator for the specific toroid I need for the frequency I am working with, calculate the required number of turns and have my real life wound one come close to the theoretical values this is perfect.
I have mainly built radios using them such as the Bitx20 and have found them to work just as I wanted them to.
I hope this helps - if you are not in a rush I would definitely say give these ones a try and see if they work for you.