MDE has turned into a very good app, and obviously needed, if you utilize the FP system to it's fullest, to easily keep track of dining reservations, Disney Resort info, MM pictures, etc. And best of all...it's for free!
I've used a few of the paid subscription apps in the past, and they really don't provide that much useful information and just seem like a waste of money. Although the people behind these apps have put in a lot of time and effort into designing them (and they do a good job), it seems like they are just using a lot of the same info that you can get on MDE for free and reformatting it. Plus, I would rather trust getting my information directly from the source (i.e. Disney) versus a secondhand person.
One app that I'm tempted to try is the Hidden Mickey app. When my family and I walk around the Parks, we like to search for Hidden Mickeys and the app seems like an easy way to help with our search. We have the Hidden Mickey book and I would like to get the app, but every time I look into getting the app, it seems like it always has poor reviews...so we stick with the book!
As for non-Disney-realted apps...
I'm pretty good at planning and utilizing the FP system, so we don't tend to wait long in lines, but one app that I do see many people using in lines is the game Heads Up! I have not used the app myself, but it does seem like it would be a lot of fun to pass the time when you're stuck in a long line. Just think about being stuck in the FoP standby line for 2.5 hours...I couldn't even imagine!
I would love to hear what others think!
* Edit * as I was typing the above EdmondD beat me to the punch with Heads Up!