I find these interesting because they're usually these big masses in the middle of a lot/street, sometimes surrounded by plants, sometimes not. Contrary to the reading, I actually have always noticed them, and have always wondered what they were...
I still remember the first time I saw one of these, I felt that they were quite clever but now they're kind of an eyesore...I've been thinking about the ones disguised as crosses, but joked with a friend that they're probably all in Texas as it would blend better into the urban landscape there.
(Unfortunately I wasn't able to snap my own picture while driving, so this is someone else's.) I find it interesting as its implementation was somewhat controversial in L.A.. I personally do not own one of the transponder boxes, but have been in a car with one. It's pretty interesting how it works--you have to pay an inital fee for the box, which becomes credit towards future tolls. They do have a system of cameras set up as well as the reciever to tell if you have the box set up. It's a weird additional cost to the commute (though I guess lots of countries do have a toll system setup on their roads to reduce congestion), and the possibility for surveillance worries me a bit...