Sorry to disagree, but it is a fully playable demo (at least the Dreamcast version is, as that's the only one I've tried).
Here's the demo release statement from the Kickstarter page...
Yesterday we released a playable demo version of Saber Rider and the Star Sheriffs to our backers. Today we want to share the demo with everybody else. You can try out the demo on Windows, MacOS, Linux and Dreamcast. Unfortunately a demo for Nintendo 3DS is not possible.
This demo represents the first mission of the game and let you try the Hero Mode. The mission in this demo got some changes, so that the game story won't be spoiled. The first mission in general is based upon the two Saber Rider series episodes "Wild Horses Couldn't Drag Me Away" and "Little Pardner". In the final version, this level will have a few more story driven events and a different end boss. The current boss in the demo will later be a mini boss.
My first impressions after playing it for a while; it's a fairly standard 'run-and-gun' affair, but it looks nice and manages to evoke enough 80's nostalgia to elevate it to something greater than the sum of it's parts.
I'm probably going to chip in to the Kickstarter - if I can afford/justify it after pay-day this month, I'll probably be aiming for the $99 'DREAMCAST LIMITED EDITION' funding tier.