I'd thought about investing in a dashboard cam, but bought a relatively cheap compact camera mount for my windscreen instead. Took this footage as a test, works very well, one or two limitations notwithstanding.
It's a fairly bulky thing with a large suction cup but even so I wouldn't use a heavy camera with it. I'm not the sort of person to record all my car trips, if I was I'd buy a GoPro or something more discreet. The only way I could get this to work satisfactorily was by hanging the camera from it upside down (so I had to rotate the video 180 degrees in the edit). But I already had a suitable camera and this mount cost less than £4, so happy days.