We wanted to be able to do some touring of the Big Island, but not be trapped on a tour bus and forced to stay on their schedule and destinations. My friend found Shaka Tours via the app so we downloaded it. I liked that we were able to choose and purchase the tours we wanted (including being able to download all of them at a good price) and were able to plan our days based on the hours Shaka indicated each tour could take.
it was a lot of fun being able to take our own vehicle, set our own timetable, hit spots we wanted, skip other we didn't (or were repeats) and the GPS stayed with us almost all the way. We did the South Island Epic Coastal Journey (incorporating the Volcano trip), Kona "Big Kahuna" Loop and North Island Jungle Adventure Loop (different days, of course). The educational moments explaining not only the obvious tourist spots but the sacred areas/stories that weren't posted made the experience educational and entertaining.
There were a few times that the voice would fall "silent" on the trip, even though we were passing spots we knew were supposed to have some historical significance, but to be honest I can't say if it was the app or human error (probably human error).
The one piece of advice, besides "get this", i would give is to make sure that you either rent a car that has bluetooth capabilities or that you have an mp3-style connector that can plug from your phone to the car's radio (luckily, I just happened to have one). You can still make it work straight from your phone's speaker, but its a lot more difficult to hear than running it through the stereo speakers. Especially if you have all the windows down to get as much sunshine as you can get!
I would recommend this for families as well as couples, friends and groups. The freedom to move at your speed and choose where YOU want to go was worth every penny.