I've only had one cop notice it, because he was following behind me on a motorcycle. I could tell it didn't really register what he was looking at until we came to a stop light. I watched him in the rearview as he leaned forward to see better, then jerked back. Took him a moment before he grinned.
As long as it isn't covering the window completely it's fine. It took me about a week to get used to having something back there, but I can see just fine through all the legs.