Quality. Their construction seems very good and they will probably last a long time (I have had them for 2 months). They are also warm.
These specific ones did not fit my foot. Birkenstocks have a textured sole that unfortunately causes slight pain in my left arch. I have a pronounced flat foot, so that is probably the reason. Also, you need to get the exact size right, because of the texture of the sole, it needs to fit well.

As a first impression, they seem pretty good. I didn't fall in love with them completely. I will give them a chance and I will not return them because I think I found them at a good price of 44.90€. They are stable and do not slip (I did not try them on a wet surface such as a bathroom with water). As a woman, I would like them two centimeters thinner, because a slightly thinner sole relaxes the female foot when walking. They have a textured sole, which I need time to get used to, it bothers me a bit for now. It will show over time if they withstand time and are worth their money. They have the correct size, I chose them because they are made of fabric and have a leather sole, and I did not have many options with these features in slippers, so that my feet do not smell from their daily use!