I am a local and have been wanting to visit Lincoln's Cottage since it opened and I finally had an opportunity to go today.
I'm not sure it's worth the full price admission. If you can go on a Groupon or special reduced rate coupon deal, it is probably worth it.
Definitely buy your tickets early as tour times sell on on weekends and holidays. We went on Labor Day and most of the tours were sold out when we went to buy our tickets online that morning.
Getting there: We took Metrorail. We walked from the subway station (about 5 blocks to the gate/guard station). At the gate, there is a shuttle that will take you up the hill to the house. The shuttle runs about every 15 minutes. Since you have timed tickets, factor in extra time to wait for the shuttle. While the walk to the cottage from the entrance gate is all up hill, it is not far, probably about as far as from the metro to the gate. They do have parking, it is behind the visitors center. It might be worth it to park if this is the only attraction you are going to see that day.
You check in in the Visitors Center Gift Shop. There they give you your tour pass and you wait in the lobby for your tour guide. You can shop before or after the tour. After the tour you get a bookmark from the Gift Shop, so make sure to stop back there on your way out.
You are not allowed to take pictures inside the cottage, but you can take pictures of the building and some cannons and statues they have outside.
The Tour: The cottage is not air conditioned. They do have ventilation, but it's very hot inside on a hot and humid day. I highly recommend going on a cool day during the cooler months. The tour lasts a little over an hour, there isn't as much walking as there is standing in each room to hear the presentation. The tour starts in the Visitors Center lobby. Then you view an 8 minute film. There is limited seating for the film if your tour time is sold out. Many stood for the entire film. After the film, you walk over to the house. The house is unfurnished. The tour focuses on stories about Lincoln related to the cottage. People who visited him there, what his day was like there. There are a few rooms where you can sit, again seating is limited. At the end of the tour you exit outside the cottage where you can take pictures or go back down to the gate to the metro.
The cottage does have an elevator/wheel chair access if you can't take the stairs.
The gift shop has bottled water, cold drinks and hot tea/coffee (Keurig) for sale.
There are bathrooms in both the Visitors Center as well as a bathroom at the very end of the tour as you exit the Cottage.
Our tour guide was excellent, I just don't think you get the bang for your buck with this tour. If you are interested in Lincoln, this tour gives a different side of him, the "person" or "human side" as they say on the tour, different from the Ford's Theater Tour (which focuses more on his death) and the Lincoln Memorial Tour (which focuses more on his speeches).
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.