Well the AF in low light isn't very fast like it otherwise is, though it at least has a focus assist beam. It has fast AF in normal conditions, and it does have great IQ. A faster lens can help its low-light ability to focus. It does very well with stabilizing low shutter speeds, and it doesn't have that mirror slap issue for slower shutter speeds as well which assists its low-light function.

The only time I would consider AF in low light "good/very good" is with the feature of a cross type phase detect AF point and 2.8 or faster lens, available only in pro-grade DSLR bodies.