I saw it the other night, thought it was pretty good, i havent seen the first one, but i dont think you really need to. It basically tells you what happened in the first one at the end of this one.
It does keep you thinking, trying to piece it all together, but at the end it all makes sense... I'll deffinatly be going to see saw 3, i assume by the end that they will be making a third. (Y)