Low light shots are quite good on the X and noticeably better than the 7 Plus. I think the improved sensor is part of it and the other part is the post-processing with the Bionic chip. I've taken test shots in a very poorly lit room and noticed a decent amount of grain during the live preview that doesn't appear in the finished shot. The post-processing softens image detail of course, but it seems that Apple struck a good balance because the finished shot looked surprisingly good to me considering the very low light I was shooting in.
I'm quite happy with the low-light performance of the X. Although of course you can't beat a DSLR with a low-aperture lens, and I do break that out for special moments.