The Strikers 1999 app just got a pretty neat update. Normally, most jets are locked, and have have a variety of ways to unlock ranging from simple (Let them post to Facebook), to tedious (Collect 3,000 medals).
There's a "random select" function you can use tokens (The in game currency) for, that randomly select any plane for one play, whether they're locked or not. This does NOT include continues, where you're stuck to unlockables.
Well, they just added a feature where, for the price of a 30 second ad, you get one session with any plane you want. Again, no continues apply, but it makes a huge difference using the transforming robot jet gun for awhile (Furthest jet to the right, and best in the game.)
With it, playing through Hard mode and racking up the 300 jewels (Premium currency) needed to unlock is pretty easy, as one full play nets about 34 jewels a piece. (You start out with about 100 jewels.)
But you only really need the jewels for the robot jet, as it's the only one without an alternative unlock method. (Hey, best in the game. You think they'd let their prize fighter loose without tying it to premium currency?)
The tl:dr version, Strikers 1999 is pretty fun if you like shmups.