If you liked the demo then get it now. The full version isn't any more buggy than that, and it has a lot more content. With the full version you can also properly hop into the MP world, and that's what the game is really about
From what I've gathered it seems vary from people to people. Some have trouble with the game, while most can run it great. I'm happy to be one of the latter, and after the latert patch haven't come across any showstopper bugs. Even the campaign played out great at my end and I didn't have to cheat my way past any missions. BIS has a track record of releasing their games buggy but eventually fixing them, and by the looks of it Arma2 is no exception. A few patches have been released already, and the next one is just around the corner.