You need to go if you can.
It's large. It fills the Hall of Flowers and spills out onto the patio. There are a few talks but it's mostly a show, and a sales area. There are vendors from other continents so you can buy orchids you won't find elsewhere.
There is no good parking. There is parking available on the roads in Golden Gate park and on the nearby streets, but it's first-come, first-served.
Depending on where you live you might consider parking at a BART park and ride, taking BART into San Francisco, then transferring to the N Judah streetcar. It stops a couple of blocks away, very close to the Hall of Flowers.
In the attached screen cap, the irregular tan building in the park is the Hall of Flowers, on 9th Avenue in the park. The westbound N Judah street car stop is the red dot a 6th Avenue. There is another westbound stop, maybe a little closer, on 9th Avenue. The eastbound stop is just west of 8th Avenue, a little closer when you're carrying a lot of plants.
Bring your own sturdy bags.