It's the most prominent feature on the skyline for Sumner Tunnel-bound vehicles in East Boston, whether driving south on 1A from the North Shore or--more relevantly--everyone leaving Logan Airport. Although I haven't driven it recently, the same is likely true for US-1 southbound drivers crossing the Tobin, and a lot of northeast neighborhoods' view corridors of the skyline.
I agree with citydweller: 1 Congress is on track to become Boston's most identifiable landmark skyscraper, an accolade currently/previously held by the Prudential Tower, 200 Clarendon (nee Hancock Tower), the Berkeley Building (The Old John Hancock Building), and the Custom House Tower.