Walter C.
Yelp
The main reason to visit this place, is the tomb itself, inside the building with the dome. The inside does not have much. Just a collection of Civil War flags, and the coffins that has the remains of Ulysses and Julia Grant. Plus the busts of the Civil War generals, located in the basement, and surrounding the coffins.
The architecture is really nice to look at, and just really grand.
There is also a visitor center, which has a gift shop and small museum. The museum has a series of descriptions on the life and legacy of Ulysses Grant. Plus a short film as well. Located across the street, towards Riverside Park, you will find it below the pavilion with columns. There are signs that point the way. Worth a visit, and something to do while waiting for the tomb to open again.
Speaking of which, the tomb is not opened the whole time, alternating hours between opened and closed. So check for hours ahead of time. If arriving during one of the times when it is not opened, you can spend the waiting time at the visitor center.
Also, the lighting can be very bad, as the tomb is very dependent on sunlight. So it is recommended to arrive earlier in the day, to see this place, and take some good photos.
It can be seen quickly, but the General Grant National Memorial is still worth a visit, mainly for the history buffs. But if you are not, then I think it can be skipped, as I don't think it is grand enough to really go out of your way to see it.