The triumphal arch is a very impressive; one really has to see it in person from different sides to appreciate its monumentality. Photographs do not do it justice. There are plenty of benches around the Plaza for resting tired feet and admiring this show of Union bravado.
For those who are interested in the American Civil War, combining a visit to Grand Army Plaza with Green-Wood Cemetery, where numerous Civil War veterans are buried, might make for an interesting "Civil War in New York" tour.
GAR Plaza is located by numerous Brooklyn attractions, so you may encounter it even if you're not intending to visit it specifically.
A Warning: Be careful crossing streets around Grand Army Plaza. Cars are coming from all directions and there are some very aggressive drivers. The streets around the Plaza are littered with the wreckage of traffic accidents.