As with most things, it depends on your use case. Except for some very high-volume or ancient features, it has pretty much everything.

However, unlike Postfix, you can actually configure OpenSMTPD yourself without breaking it. Postfix suffers heavily from a Byzantium configuration that everyone is copy-pasting because understanding how to create a configuration from scratch requires man-months of reading their docs.

The biggest downside to OpenSMTPD is the portable version is weakly supported, so it's only useful if you are running it on OpenBSD, which is a shame, really, because it's probably what most people would prefer if they could run it on their preferred OS.