You will notice it in the steering wheel. It will prob be slightly different for each car but I know in mine there is a slight rattle of something coming from inside the dash at low rpms. My drivers side mirror also vibrates at idle but stops as soon as you get moving in a gear. The mirror on the windshield also slightly does but not that bad.
It's also a little noisy but you have to figure the point of the mounts is to not lose power due to engine movement so stiffer mounts means there will be more vibration sent to the chassis.
03 Ford SVT Focus - Toms tune, Steeda mounts & stress bar, ORP, 66MM BBK TB, CFM UDP, Rota Grids
The Bad Cat (97 Merc Cougar XR7) - Flowmasters, MSD ignition, K&N intake, 19" Stern ST-4 Warps