Its really well worth the money.
The calculators- easily see how much of what you need to get from level X to level Y
GE Search tool- can look up any item on the grand exchange and see the current price of it, along with the price of it over the past 30, 90, or 180 days
Quest guides- very easy to see what I need to do if I ever get stuck at any point in any quest. Will probably quest a bit more now
Monster info- you can look up monsters to see what the top drops are (King Kalphites top drops are Drygores for example), what its weak against so you know what combat style to use against it, and where to find it. Very useful to see what I can get from various monsters.
Alching Guides- it tells you what you will get in profit if you alch something, and how many you can buy and in what timeframe
And so much more!
Sure there are websites that could also tell you all of this info, but there isnt one that does it all. Its a lot more convenient having it on my iPod than flipping back and fourth through pages and waiting for the page to load. This app load all of its data almost instantly
I highly recommend it to anyone who has well invested their time into Runescape