I rarely use zooms, and long focus lenses; my preferences go from ultra wide to normal primes. The Pentax SMC 75-150mm f4 is a remarkable exception that I prefer to keep in my collection when I need something longer. One of the best vario lenses I’ve come upon, it keeps a constant f4 aperture along the whole 75-150 focal range. Very sharp, and relatively small, especially compared to modern telephoto zooms. The bokeh is nice, the varying short tele-to-long length makes it ideal for portraits and details when you walk around. Produced until 1985, Pentax claimed it achieves the goal to have vario lenses with the quality of primes.
The biggest negative thing about it, and one that cannot be avoided as well, is the zoom creep. It is convenient to have one-touch zoom, on one hand, where the zoom ring is also the focusing one. On the other, one is hardly able to stick the lenses focus-zoom it into a position, especially if you point upwards or downwards. That makes usage of a tripod with it almost impossible: the ring always slowly creeps into one direction, depending on gravity, changing the focal length in the meanwhile. Yet I’d prefer it above the SMC 200mm f4 one that I have as well, as the 200mm is a bit too long for my taste, especially on a 1.5 crop. Definitely the 75-150mm is a must keep for me.
All photos taken with ISO 400 on Pentax K20D.