Left-handed usage - It is very easy to change from right to left, you just place the ruler on the other side in its trench. Works the same way. My sister is a lefty and she uses it too.
The safety is not a huge issue because when you are holding the ruler with one hand and cutting with the other, your holding hand is actually about 1/2" ABOVE the cutting surface. Much safer than trying to hold the ruler and cut. Of course, if you are like me, you can still find a way to get cut -- been there, done that, had the stitches to prove it.