The DLC is good. If you ever downloaded the Solo pack on either Super Stardust HD (PS3) or Super Stardust Portable (PSP), then you have an idea what to expect here. It's got the following modes:
Endless: One life, and one continuous game with enemies frequently spawning into the playfield.
Bomber: One life, and bombs are your only weapon.
Impact: Now you have no weapons at all. Instead, you destroy enemies and obstacles by boosting and using the resulting temporary boost invincibility to ram them.
Twin-Gun: Similar to the standard Planet mode, except you can fire two weapons simultaneously. Each of the Vita's analog sticks controls the directional aim of a weapon (like the right stick does in most modes), and tilt control is used for ship movement.
I got the DLC pack in a bundle along with the main game, which is cheaper than buying the game and the DLC pack separately. If you don't have the game yet, and those modes sound interesting to you, then I'd say go for the bundle.