<p>I owned the 35 2.0 since 1990 and upgraded to the 35 2.0 IS last Fall. Both are nice lenses but the 35 2.0 IS one ups the older design in most ways save for cost: sharp edge to edge at all apertures, silent, faster more surefooted focus, 3-stop IS (Canon claims 4) and a more handsome appearance. Both lenses are very sharp in the center but the newer design has much better edge performance and less CA. The focus motor of the old 35 2.0 is buzzy and, worse, has trouble locking in low light.</p>
<p>However the biggie for me has been ease of shooting. With the older 35 2.0 I had to look for points of contrast to lock AF in low light, so lots of lock-AF-recompose technique. With the new 35 2.0 IS, it pretty much locks on anything, at least with my 6D, and I can shoot from the hip without worrying about doing the lock AF recompose dance.</p>
<p>I also have the 40 2.8 STM and while a bargain for $150, it pales before the 35 2.0 IS for low light work.</p>