I've had a Driveway Patrol unit for a couple of years or so.  It works well for the money I paid, which was less than $20, including postage, if I recall correctly. 

The batteries last a looong time.  I've only replaced the batteries one time since I've had it.

I have the unit just inside a gate inside my back porch area where it's sheltered.  It's mounted about 3-3.5 feet from the ground.  I want to know if anyone enters my gated area.  The chime unit is just inside the back door.

The chime has three settings, Off, Low, High.  The low setting his normally loud enough to hear in our entire 1700 sq. ft. house and is heard audibly from the gate and eye, hopefully deterring the intruder.  The eye and chime are 15'-20' apart and on opposite sides of the back door.

The unit was very easy to set up.  It took only minutes.

The eye has a wide angle of sight, which would be good in many settings, but having it inside our back yard gate, I don't want it to go off when my family is using the back yard, so, I taped a piece of paper over the half of the eye facing my yard and that part is fine.  I have two small dogs that regularly set off the alarm when they go up to the gate.  I could probably remedy this by tilting the mount of the eye unit up a little?

Overall, I'm happy with the Driveway Patrol. 

If the company that makes it ever upgraded it they might add a button to it that would temporarily disable the alarm for, say, 5 minutes.  That way when we take out the trash or when we're leaving the house, we could push the button and not have the alarm going off at those times.