There are loads of apps that claim increase mental sharpness, memory and other positive aspects of memory, but from those I've tried, they felt like games, instead of being a material to learn from. They even market themselves as games - because "learning should be fun". As such, the results for me were poor.
For example, I've ran into an app for mental math skills, and while it does provide excellent tips, the core of it is a time-bound game in which the player is supposed to answer or fail. I am a slow thinker, so you can imagine how that goes. If those tips for mental math were interactive, it would have been an excellent app, but it just dishes out numbers and asks me to perform the calculation - something I don't even need a pen and paper for - and puts a time limit so I feel bad for being slow.
Have you found some apps that you can definitely say have helped you with mental skills? What makes you say that? Which positive results do you see/hear/feel?