I mildly disagree with the previous post, at least regarding LGA buses. In the afternoon and evening, they have no special lane or any other advantage. They sit in stand-still traffic just like everyone else, while only one lane goes in-bound to Manhattan through the Midtown Tunnel. At those times, the subway is a big winner, although it's awkward with luggage.

The buses from JFK generally do all right. If your ultimate destination is reasonably close to Grand Central, why pay extra for a taxi?

Bruce