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.