I've owned one since 2011, and have found it be be surprisingly accurate. It does not have GPS, but simply counts steps, and based on your "profile - length of stride" calculates distance. It does sense altitude change, and I also find this function to be accurate. I've compared it to Garmin GPS as well as iPhone GPS, and the results are much the same.
The phone app (either android or IOS) simply gets the data from the fitbit via Bluetooth, and if cell phone coverage is available will sync with your page on the fitbit web site. It also displays your information in easily understood formats. There is also a "dongle" that plugs into your computer USB slot for syncing without the phone app.
The web site is very good. It keeps track of your hiking activity and presents a good user interface to display your history. You can also enter different types of activity (bicycle, swimming, etc.....). It also allows you to record your meals (as you enter more data this becomes much easier since your "favorites and recent entries" are easy to "click on" without having to enter much data), and calculates % fat, protein, carbohydrates, etc.... They do have a fairly comprehensive database of foods as well.
Using the food information and your exercise information it will tell you how your caloric intake compares to your exercise, and if you will likely gain or loose weight. You can set goals and it will track your progress toward those goals.
The main advantage over the iPhone health app is the web site. Without the website I would say do not bother. Now, with the Apple Watch I fear for the fitbit. If Apple, or one of their partners develops a good website I think the fitbit will fall behind.
As it happens my 4+ year old Fitbit ONE just stopped working (Bluetooth flamed out), and I do not intend to replace it. I will just use the free iPhone Health App. I could still use the Fitbit web site, but will probably not go to the trouble.
Oh, and no heart rate info.
Hope this helps.
Edited by John Sims (05/19/15 06:55 PM)
Edit Reason: Heart rate comment