I've gone a few times, and honestly it's pretty much the same thing you will get at Knotts, just a little less expensive.
I've always found that your experience will depend upon the mood you go in with. If you are paying attention, of course you can tell all the props are fake and you can anticipate when and where things will pop out; but it you have a fun attitude about it you can certainly enjoy yourself.
As far as the old zoo is concerned, it's there year around for you to hike to/check out. Lots of the old cages are out in open in the park area so they aren't particularly scary. In my opinion, as someone who was born and raised jn LA and lives walking distance from Griffith Park, it's much creepier not all decorated for Halloween.