Although most languages now are not boustrophedonic, there is at least one partial exception: Japanese. If you look at the right sides of company cars in Japan and you can read kanji, you will notice that often the name of the company on the right side is written backwards.

So for example, if the company is called say "den ki", then on the left side of the car near the front there will be "den" and towards the back there will be "ki", and that's normal. But, on the right side of the car, the company wants to have the first part of the name nearer the front of the car, so they will write it starting at the front of the car again, reading left this time. However, the left-right orientation of the kanji character itself will not be reversed.

Which brings up something pretty interesting - which level of rearrangement of a written language would be most efficient in boustrophedonic form? The simplest way would be to just use mirror versions of each letter. Next, you could use the correct letters, but write words from the right to the left. Last, you could write entire words the normal way, but write them backwards. Here are three forms of "Some sentences are easy to read, and some are difficult."
  • (unwritable here)
  • .tluciffid era emos dna ,daer ot ysae era secnetnes emos
  • .difficult are some and ,read to easy are sentences some
To my eyes, the third one seems the easiest really - all of your word recognition abilities are preserved intact, and the only change you have to make is the direction the eyes move.

The illegibility of the 2nd example above indicates something about the way our brain processes things.

Where would the first example rank in legibility?

The sides of cars are the only times I've noticed this phenomenon in Japan, otherwise they usually follow either left to right (in modern-feeling things) or top right down, in novels/traditional things.

Also, japanese is a special example, because during most of the evolution of the language, it was written top to bottom, and people still spend massive amounts of time practicing word recognition in both forms - most school books are written western style, but every japanese student also reads lots of japanese literature in the process of learning the kanji, and it's all written top to bottom.