I am just finishing up the builds for the demo and the first release version. I don't want to call it an alpha or a beta, but it is a pre-1.0 release, and given the version number 0.1. All the features I have planned are in the game, and the game plays smoothly, but I am sure I will continue to polish and tweak things as needed. Purchasing the game gives access to all future versions and updates, and unlimited downloads for Windows, Mac and Linux.
I'd recommend trying out the demo before purchasing to get a feel for how the game plays. The game seems to work well on Windows, Mac, and Linux (tested on Ubuntu LTS 12.04), thanks to Unity's cross-platform support. It's a lot of work building and testing three demos and three release versions, but everything seems to be working.
I'm looking forward to getting some feedback from some of the first people to try out the game. I think a $2.99 price is fair for the quality of the game and the scope of the gameplay. I still think it is fun after many hours of play testing.