But you need a lot of extra RAM compared to the usual filesystems. Is that correct? If yes, is using ZFS worth the cost of the extra RAM?

You don't need the extra ram. ZFS uses it as a cache, more ram, more cache. That's about it, otherwise it doesn't use much ram.

ZFS doesn't use that much RAM if you don't turn on deduplication, which is of less use in a system that serves large compressed video files than one that handles tons of read/write traffic as an office file server.

RAM is not required; it just makes lookup faster with caching.