I really like this app a lot. I'm a terrible procrastinator (meaning the opposite, of course; I'm really good at procrastinating) and this app saves me from myself.
OneFocus is simple to use and does what it is supposed to do well. The developer is very responsive to feedback, which is great.
I should say that with any app like this, there is a moderate amount of trial and error on the user's part as you figure out which apps really need to be running. I've found it helpful to select short amounts of time as I figure out how it works best for me. Of course, all of this is not a critique of the app itself, it's just part of learning your workflow with a new app like this.
Overall, really great app.