I can't make up my mind about the GPS issue. Like yourself I find it extremely intrusive, and I do have a problem with the idea that they can attach it to your car - are you really 'secure...in your [] effects' under the 4th amendment if the police can mess with your stuff in this fashion?
On the other hand, we accept the reality that the police often stake out suspected criminals to observe where they go,, who they meet etc. So if a police officer was watching someone's house from across the street, watched the resident emerge and drive away in his car, and followed the suspected person with a tail car or a police helicopter, we wouldn't find anything particularly unusual about it; when you're out in public you're subject to being observed by law enforcement as well as everyone else.
So in that sense, the GPS is doing nothing more than automating what would previously have required a team of undercover officers to observe - just another instance of modern technology creep. But it means we now gather information on people where we might not have been willing to commit the resources before unless it was serious enough. And there's the issue of where the the line between observation and interference is.