Gamedaily has the demo:
http://www.gamedaily.com/download/in...eid=0078701454
I've been playing the demo on and off (RL has kept me busy lately).
The demo comes with a "mini-manual" which only explains the controls on the various screens, and a "tutorial" mode which is just enough to get players started. The lack of documentation was frustrating at first, but I eventually found the answers to most of my questions at the Kerberos forum, which has a whole sub-forum dedicated to the demo. Nevertheless, I suspect some potential players may have been put off by the lack of handy instructions.
More information on the game can be found earlier in this thread, and in the thread "Sins of a Solar Empire" also on this forum. For those undecided about buying SoTS, I strongly suggest investing some time in playing the demo; that would be more useful than any second-hand account I could give.