[kyrra]

It is optional to use. Just like it's optional to use Googles Dns servers at 8.8.8.8 and 8.8.4.4. The advantage with this is the extra security you get with HTTPS.

Google DNS tends to be one of the fastest DNS servers you can use (just benchmark them against other options). The IPs are anycast, so you will likely be served by the Google data center closest to you.

As for what they log, check it yourself: https://developers.google.com/speed/public-dns/privacy