I have flagstone perches and I love them. Very easy to clean and they dry fast. Chins absolutely love them too. Perches are not slippery.
But there is a gap between the stone and the cage, up to 3/8''(depends on the perch). If you worry about this ask John not to install hex nuts between stone and a washer.
And I will not buy a corner perch again, the one that I got did not fit .I called and asked for replacement. I was told to put one side on one level and another side higher or lower. I refused, because this way I would end up with more that 1 inch gap. Then John suggested to hit perch with a hammer, so I can have 2 regular perches. I did not agree ( who knows maybe it will break into 4 pieces not 2 and I do not need sharp edges) so I insisted on replacement (regular perch). I paid for shipping because return label never arrived, but John sent me extra manzanita perch (same shape as stone perches), that was nice.
Also be specific about the shape you want. Some perches are long, but narrow, it's uncomfortable for chins.
I like medium size, XL is soooo HEAVY.