I bought it to use the many Pentax K-mount lenses (of various generations) that I use on the Pentax KP, also on the Sony A7r 2 that I recently added to my kit.
The adapter is well made and consists of very solid metal for the structure, for the connector and for the bayonet.
The notched ring of polished metal visible in the center allows the diaphragm to be closed between the maximum and minimum in 11 positions without reference. In principle, you can close the aperture of any lens, but you don't know exactly what value it is set to (except of course the two extremes, which correspond to the maximum and minimum of the mounted lens). This obviously applies to the most modern optics without their own aperture ring.
The rotation of this ring, which basically wants to simulate the membrane ring, is quite fluid and has small shots to facilitate positioning as much as possible.
I've used it with various Pentax APS-C lenses, from the old 50 M 1.4 to the Sigma 18-35 f/1.8 Art and 17-70 f/2.8-4 C.
All work perfectly, always with the limit , which imposes an approximate selection of the membrane on the most modern models. Obviously there are no electrical contacts, so there is no Exif data on the optics and focus is manual only (but the A7R2 is excellent in this regard).

The very need to focus manually and without an electronic connection prevents the use of some modern lenses, that is, those in which the focus ring does not act mechanically, but via the camera module.
For example, the DA 18-50 WR RE, a very versatile and lightweight lens that I personally value very much, is unfortunately unusable because there is no way to focus.

Apart from this limit, which the adapter obviously cannot resolve, the product is very valid and allows you to capture excellent quality images from optics designed for another brand. In particular, the Sigma Art 18-35 still offers spectacular photos even if you use the APS-C crop mode (although the A7R2 sensor's 42 Mpixels allow you to take 18 Mpixel photos, which are still valid).