I use this daily even as I myself am making my own Pomodoro style app, it's a case of me procrastinating shipping my product so I tearfully use someone else's instead in the meantime (there's a good blog post about this by Kitze, with HN discussion [0]). My product [1] basically solves a lot of the issues I have with regular pomodoro timers, such as that the work and break times are rigid between tasks and you can't add or subtract time; that you can't export the times as a PDF or CSV to send to clients if you're freelancing and billing by the hour; that you can't organize tasks in a calendar-like format, et cetera.
But until I build that, I'll have to continue using Pomofocus. It has a great free plan, I wonder how much money the founder makes from a B2C product.
Also, something I learned recently was that Pomodoro is trademarked, which is why you see most timers riffing on the name otherwise calling it something else.
[0] https://news.ycombinator.com/item?id=31352108
[1] https://getartemis.app