On a Roll is a basic Sonic the Hedgehog sendup. You play as a rolling ball tasked with finding your way through sixteen levels of enemy balls, spikes, and Sonic-like terrain. You collect stars of various values (rings) on your way through to protect you from harm--every time you hit spikes or an enemy ball, you cough up 100 stars. Don't have 100 stars? You lose a life. Don't have a life? Game over and back to Level 1 you go. Supposedly there are bosses every four levels, but I could only make it a little of the way through Level 3 in the four tries I gave it (there are no continues, so game over really is game over).
The enemies and spikes aren't so bad, but the drop-offs are really difficult. The weird momentumy-rolly-physics makes stopping on an edge a bit of a chore, and if you even breathe on the joystick at said cliff edge you're going over into the abyss. The stars do nothing for you if you fall into a pit, which really makes collecting them a futile effort.
Even supposing you did manage to build up a good store of stars without dropping into a pit, your stars are reset between each level (even though your life total carries through from level to level). Even at a reduced 80-poit price tag, On a Roll is only a borderline good deal for the hardcore Sonic fan who's jonesing for a bit of cheap nostalgia. For the rest of us, it's pretty awful--skip it.
If you really want to try the demo, do yourself a favor and mute the TV before you open the game up. On a Roll has quite possibly the worst music of any indie game currently on the market.