Yeah, it's a tradeoff. Zfs gives you easy data integrity verification (and recovery if you have redundancy), easy snapshotting, easy send/recv. But you lose out on modify in place, and unified kernel memory management (at least on FreeBSD and Linux, maybe it's different on Solaris?); both of those can reduce performance, especially in certain use cases.

IMHO, zfs is a clear win for durable storage for documents and personal media. It's not a clear win for ephermeral storage for a messaging service or a CDN. If you don't mind running multiple filesystems, zfs probably makes sense for your OS and application software even if your application data should be on a different filesystem.