DL used to prohibit any type of connections to/from Shuttle flights in BOS/LGA/DCA. However, they have since changed the policy and now allow these connections. Obviously, connections are very very easy to/from in BOS and DCA... LGA is a bit of a challenge, but the shuttle bus runs every 30 minutes and is timed to leave/arrive just before/after the shuttle flights on the half hour. Only potential jam is the line for security at the main terminal...but if you have status, it is not too bad