Not entirely true. While there was a lot of skepticism on AdSense and Gmail within Google, Brin was really enthusiastic about both products. If I recall correctly, Brin was even the promotor and driver of AdSense ("Let's get this puppy launched", a famous quote by Brin on the launch of AdSense). Both Page and Brin were also the first users of Gmail and tested the product intensively within the company after Buchheit got approval for launching Gmail internally.