I have played the 'tutorial' mission and it was quite fun! I bought the game and started the next mission (campaign mode). I'm at level 2 now and wonder how many levels exist in the current Version(0.9.8)?
Factorio currently comes with two campaigns, "First Steps" (3 levels, the tutorial) and "New Hope" (4 levels), for a total of 7 levels. (Internally, these are called "demo" and "beta".)
It also has the Scenario Pack, which contains two more "side campaigns", Transport Belt Madness (3 levels) and Tight Spot (5 levels) -- giving you a grand total of 15 levels.