The impact can be seen from the tons of positive reactions to ethstaker.tax announcement posts:

https://www.reddit.com/r/rocketpool/comments/1ak6w82/ethstakertax_now_supports_rocket_pool/

https://www.reddit.com/r/ethstaker/comments/1ak6wvi/ethstakertax_update_rocket_pool/

https://www.reddit.com/r/ethstaker/comments/10p0bum/ethstakertax_news/

I don't like to do invasive tracking of the website usage, therefore I can only say there are tens (30-50) of daily users of the website based on the amount of API requests it handles. While this may seem low, it is important to consider most stakers only really need to use the website once or twice a year.