ATARU, starring Nakai Masahiro, Kitamura Kazuki, and Kuriyama Chiaki, was reported to have achieved an average viewership rating of 19.9% for its first 1 hr 20 min long episode. The highest instantaneous rating was 21.7%, which happened at four points in time -- 9:49pm, 10:09pm, 10:10pm and 10:11pm.
The drama received many positive reviews, demonstrated by "ATARU omoshirokatta" (ATARU was interesting) becoming the top trending topic on twitter in Japan for a short while after the drama ended.
Source: Yahoo News Japan, twitter
It's an excellent drama, watch it if you can!