I drive around with mine all the time, especially to/from work, where I cycle between Charles and PG, depending on where I'm at. I hear PG more, since I have to go not just through them, but into VA and Alexandria as well (Alexandria is really quiet it seems...have to check my programming), but several times, I've heard of lookouts posted for vehicles. I figure, if I happen to see that vehicle and no patrols following after it, I could always call in and let them know where it's at.
Now if I ever get pulled over for something, I'm not sure what the cop is going to think when he sees it and knows I can hear him, especially MSP...