I got this lens used in perfect condition for half the price, great deal.

It is amazing. From wide angle to portrait closeup. Constant f/4 with constant light when zooming. Face and eye detection continous autofocus works great if the person moves slowly. And the best thing: zoom ring is super smooth and it is PARFOCAL!!! A fast zoom for action scene or a slow zoom for a poetic scene can be done.

I think there is no best alternative to it. The combination of a good enough f/4 constant aperture with a complete zoom range, smooth zoom, parfocal and good continous face/eye autofocus, in a small and lighweight lens is just perfect.

It is not a great low light alternative but fast prime lenses are made for this, and a 2.8 zoom will not improve low light so much.

You can ask about shallow dof. Yes there is some background blur in 45mm f/4, enough to priorize the person face, but not too much. Shallow dof is not the main purpose of a zoom lens. This lens is great for video zoom, face focus tracking, gimbal and slider use, all focal lenght needed.