Down side, is you now have a dedicated saw to the track. Why? Cause it now has slides on the side that need a track. Plus side, this one is pretty accurate to some of the simple ones I have built over the years. Notice the blade on the saw, its fine tooth one (I think 30 teeth?) which is good for detailed smooth cuts.
http://www.instructables.com/id/How-to-Make-a-Track-Saw-Guide-for-a-Circular-Saw/?ALLSTEPS