Ueno Park is my third top spot for cherry blossoms in Tokyo (Chidorigafuchi and Meguro River are my top two). We came here 3 times in late March and early April since we were a bit early for the cherry blossoms when we first arrived in Tokyo. We traveled back to Tokyo from Osaka on April 6th to see the cherry blossoms at peak, and Ueno Park was one of the three places we visited that day.
Although I usually like to avoid crowds, one of the reasons I like this spot is so I can be among the hanami crowds along the cherry blossom lined path. It's a spectacular sight to see so many people in one place for the cherry blossoms. I read that the cherry blossoms at Ueno Park are usually ahead of other places in Tokyo, and this was the case when we visited.
The path leading up to the temple and pond is lined with food vendors. It's fun to see and try the food vendors, but it's overpriced for okay food. The swan boats at the pond are nice to see, but for a more scenic cherry blossom boat ride, I recommend Chidorigafuchi (see my separate review for that).
Ueno Park also has temples, shrines and a zoo but we didn't make time to explore them as we were there for the cherry blossoms.
During cherry blossom season you may also find there is a line up to get out of Ueno Station (the exit that is closest to the cherry blossom tree path).