I'm having a bit of trouble figuring out the point-at function.

One thing that isn't really clear is whether object orientation is important, or if it is based on the objects' relationship when point-at is set.  I assumed it was based on the z-axis, but when I try to align props they always seem to flip around unpredictably.

Does point at work with limits/can axes be constrained.  For example, could I make a turret that pointed at an airplane with movement on just one axis, and then have its guns point at the airplane with movement only on another axis?   

I am trying to have an object always point at the sun, but I do not want it to tilt or roll.  Is this even possible?