So I would say that it's currently "technically" a paid-for app. It seems like they are doing something like Sublime with a "nagware" approach. I have been using it since it was free and haven't actually needed to pay for it yet. Having said that I will probably pay soon anyway because it is such a valuable tool for me.

As for using Fork vs other apps, I used to use SourceTree before Fork. Fork is my favorite right now because it is less opinionated and really just exposes the Git functionality directly. For example SourceTree made it impossible to force-push because they didn't believe you ever should.

There is another class of app for Git that is like a subset or overlay, like the way GitHub desktop uses "sync" instead of real Git terms. I remember Tower also felt like it was trying to make me see things the way it wanted me to and not the way Git worked but I can't remember the details.