For those of you placing the transponder somewhere other than the windshield, and possibly those who are putting it behind the shaded black area next to the mirror -- mine is in the latter, and I showed a friend of mine that it works reliably there. He mounted his in the same spot in his car. Today, he received a ticket from CHP because the transponder wasn't visible from the outside for enforcement purposes while in the 680 HOT lane.
Note specifically that the wording of the violation (23302) means even if we are exempt from the tolls due to the type of car we drive, the transponder must still be "visible":
If a transponder or other electronic toll payment device is used to pay tolls or other charges due, the device shall be located in, or on the vehicle in a location so as to be visible for the purpose of enforcement at all times when the vehicle is located on the vehicular crossing or toll highway. Where required by the operator of a vehicular crossing or toll highway, this requirement applies even if the operator offers free travel or nontoll accounts to certain classes of users.
If you're using one of the newer, smaller transponders and it is sitting in the shaded area to the right of the mirror, make sure at least some of it is visible from outside.
Given that we can't place it anywhere else on the windshield and have it work reliably, I feel this is a lose-lose situation. He's probably going to end up trying to fight it but it's not worth paying a bunch of money and taking time off work for a $250 ticket that adds no points to your record, either.