Definitely a must see attraction if you visit LA. Fantastic view of LA up here. Hollywood sign to the North, and Downtown LA to the Southeast.
Entry to the building is free with lots of stuff displayed about the observatory, and make sure you walk up to the top of the building too and snap some stunning photos.
Weekends are a little chaotic to find decent parking, so head up during the week instead. If you go at weekends, the car park at the Observatory gets packed quickly, so drivers are directed to park on the twisty road nearby meaning you could end up walking uphill for over 1 mile to the observatory which is what I had to do. Still, good exercise.
Pack walking shoes too in case you want to check out the hiking trails nearby too.