I recently got a circa 1978 Soligor C/D 650mm f8.5 mirror lens for cheap at auction. Its has a T mount, and after a cleaning and adding a T-PK adapter, it worked perfectly.
Mirror lenses are not for everyone. Or anyone. At least not these days. They are generally light weight, slow, lack sharpness, and offer busy bokeh. But if you can get one cheap, they can provide some nice images under specific circumstances.
Here is a head to head comparison of a good, and the smc Pentax A* 300 f/4.0 is best in its class, and the Soligor. These are houses on a ridge several miles away. The Soligor shot is a 100% crop, and the A* 300 is scaled ( lanczos ) to the same size.
The Soligor C/D 650mm f/8.5:
vs smc Pentax A* 300 f/4.0 @ f/8.0:
There was a huge amount of atmospheric distortion in both shots. But despite the A* 300 being magnified by more than 2x, it shows much more detail. And the Soligor shows the severe vignetting characteristic of mirror lenses.
So why would you use a mirror lens? Well, gosh, they are cool! And despite the optical issues, they work very well where resolution doesn’t matter so much, e.g. video.
They are also great effects lenses. For example, if you are trying to achieve a “Its 10:42 pm and I just woke up in a mental hospital” look…
Or angels become demons:
My Soligor 650mm examples gallery
My smc Pentax A* 300mm examples gallery
The A* 300 f/4.0 is a real special lens, especially when stopped down to f/5.6-f13. But it cost me about 5x the cost of the Soligor, and does not help bring things in as much where resolution is less relevant.