HN user
Obviously having discipline, exercising and not eating junk food is a big part of it, and for many people that would be enough. What Exist helped me with on this was keeping me motivated over a period of time and helping me identify other things that were / are impacting my progress (e.g. sleep, work etc). Clearly its not for everyone - but it is certainly something that I value, and something that has helped me.
I use it. Its great for bringing together data from various self-tracking sources and try to get some learning from them.
Why - several reasons. It means I go to one place to look at this data, and its presented in a consistent way. It also helps try to drive meaning from all this data by looking for correlations and commonalities.
Did it bring improvements. For me its helped me get in shape, its helped me find things that have a knock-on effect to my mood and/or productivity.
I've used Exist for a few years and love both the service and the way its run. The design of the app is great, its reliable and continues to evolve, but the biggest thing for me has been how open and transparent the dev team are.