For myself, my pre-existing collection of primarily Fujinon M42 prime lenses was one of the reasons I moved from Fujica to Pentax. I would not have been able to justify (even if I could afford) the expense of replacing the various wide-angle, wide-aperture and specialist lenses I'd accrued in the previous 20-30 years and, for what I want, my 16mm fish-eye or my 35mm f/1.9 (to name just two) continue to produce the results
Using second-hand M42 prime lenses has allowed me to expand my horizons to places I would only have dreamed of had I restricted myself to more modern lenses! What price "triplet bokeh" if it wasn't for the ubiquitous Domiplan?
On full-frame, some of the older lenses may show their limitations, especially in the corners of the frame, but in our modern world of APS-C sensors these limitations are frequently barely noticeable!
Do try to find a "genuine Pentax" M42-PK adaptor, I know they're overpriced these days, but they are so significantly superior in machining tolerance to many of the cheap alternatives you can fit and remove them without a tool, just using a fingernail!
Good luck