Is Canon 35mm f/2 IS worth it over the non-IS version?

If you think a monopod would be useful for what you shoot, then IS would be useful. That's pretty much the easiest way to consider it. IS tends to be more useful for longer focal lengths than for shorter ones, but everyone's handholding capabilities and the lighting conditions they want to shoot in are different.

Also be aware that the two lenses are not identical to each other with just one feature difference; and the optical quality (on full frame) is not identical. The IS model is quite a bit newer and has a newer optical design. The EF 35mm f/2 is from 1990. Its design looks like this:

The EF 35mm f/2 IS USM is from 2012. It has a USM motor, as well as IS. Its optical design looks like this:

Note that it as additional corrective elements and different groupings for better optical performance. According to the the-digital-picture.com review, there are multiple improvements over the older lens:

Replacing the 22-year-old Canon EF 35mm f/2 Lens, the Canon EF 35mm f/2 IS USM Lens delivers much-improved peripheral image quality, adds image stabilization and Ring-type USM AF, and has a significantly-improved build quality - including a much better MF ring than the replaced 1990-era design. The 35 f/2 IS is modestly heavier and larger (but still small and light) than the non-IS 35 f/2. A significantly higher price tag is the big downside.