Very good, though not perfect
This app has a number of nice features, such as a timer, which helps you track how many words per hour each session of writing is. The graphs and statistics summaries - which show you how you're doing over time - are also very useful, and the app's design is simple and attractive. You can export your stats as a CSV file.
The app hasn't been updated for a long time, but I've used it for several years now and am pleased with it. It's a shame that the app isn't updated regularly, because this is one of the better writing apps I've encountered.