I just heard about this product a few days ago at Et Tu Jen? and knew it was worth a try for our family. I ordered it immediately and it arrived yesterday. I'm very pleased with it so far.
This is a little window-cling lens, about 5 inches x 8 inches in size. You attach it to the inside of your back window with water (easy to remove if needed!). What is does is magnify a small segment of your backwindow so you have a better view when using your rear view mirror.
We have a large 15 passenger van with very poor visibility in back. While there's no substitute for knowing where your children are, this product (for only $20) gives significantly improved visibility for those things you can't know about all the time - like a neighbor kid riding by or whatever.
Here are a few numbers and tidbits from our own situation to give you a sense of how it works. We have about 75 feet of driveway behind our van when it's parked in its usual spot. Looking in my rear view mirror, I can see the road, but not the driveway (naturally I can see a bit of the driveway in the side mirrors, but it's not a complete picture). One time a shortish sports car pulled in behind me when I was about to back out. If I hadn't checked my side mirror, I wouldn't have seen them at all. That makes parking lots a little unnerving at times. This lens allows me to view almost the entire driveway (if someone is holding on to the back door, you won't be able to see them, but starting at just a few feet back is a huge improvement).
In the middle of the lens, things look farther away than they actually are. Keep in mind that things at this distance are best judged through the window itself.
Yesterday, I was in the van waiting at a train crossing. The car behind me looked like it was right on my tail, but in the lens I was able to tell that there was an entire crosswalk in between us.
For more info, check out http://www.safeview.com/