Just a couple of things about the shuttle concept. The seats cant be reserved until you check in, the tickets with the exceptiion of a few discount tickets can be used on any flights. I have seen both US and Deltal add another "section" (meaning plane" to a shuttle flight when they had too many people to fit on one plane.

The shuttle designation only applies to BOS-LGA, BOS-DCA and LGA-DCA flights on those carriers. They US and DL flights leave an hour apart on each carrier but 30 minutes apart when you look between carriers. So US leaves from LGA on the hour, and DL on the half hour so you are never more than 30 minutes from a departure. US at least is still recommending a 20 minute minimum check in time. And you can really get from the check in to the gate in that time usually. The fare structure is greatly simplified. It is a bit over $400 round trip. Nothing about minimum stay or anything. They do have discounts on the weekend. While $400 may sound cheap, it is about a 35 minute flight between LGA and DCA. In addition, It 4 weeks from today I want to buy a shuttle ticket it will still cost me $400. I could possibly fly to London for half as much 4 weeks from now, so it really isnt all that cheap. At LGA but the Delta and US shuttle have dedicated check in for shuttle only (DL is in a separate terminal that is easy to get in and out of), largly dedicated waiting area with business center type facilities in some cases. Basically meant to get people between these cities fast, without much waiting anywhere on the path. The seats are leather, altough coach sized. Also with US at least if the weather is good they deplane in both LGA and DCA from both the front and back of the plane to get you on your way quicker.