No one really needs a 120mm lens. Same goes for 150mm. The field of view is just a wee bit tighter than a 135mm prime, so a 135mm can accomplish close to the same shot with a few steps forward. I'm a prime lens afficianado and so zooming with my feet is what I do to achieve acceptable framing if the lens of the day isn't quite the right choice. There's never been a time where I was out with a 135mm, or a 100mm lens, and said, "Oh boy, if I only had my 150mm, this shot would be perfect."
However, 150mm is a unique focal length, and therefore the collector bug in me mandated its acquisition. The lens is very similar, but slightly larger than the M135f3.5 lens which I haven't reviewed yet. Both of these lenses have a built-in hood, similar specs, and handle about the same. These M-series lenses are so small already, yet they manage to cram in lens hoods into many that need them.
The lens is 290g (0.64#) and, focused at infinity, 75mm long (2.9 inches) and has 6 aperture blades and 49mm filter threads. The aperture of f/3.5 is about average for lenses of this focal length. The lens bears the usual aperture ring and has a rubberized grip for easy manual focusing. Minimum focus distance is 180cm (5.9 ft) producing 1:10 reproduction ratio (0.10x).