I really don't like the steps requires for the iris scanner to unlock your phone. You first need to use the power key to turn on the device, then swipe and then put the phone in front of you and use it without glasses or contact lenses to unlock it. It's terrible usability wise. It's use the fingerprint sensor at the back. The all idea is not just to make it more secure, but to allow a more convenient way to unlock your device. I thought that the phone will unlock itself when you just put it in front of your face without needing to do anything. This would be a really useful feature. I guess that the iris scanner camera is not working on low power mode and it turns on only when you launch up the iris scanner app which launches with the 4-digit PIN code interface.
You also need to hold your phone pretty much very close to your face in order for the iris scanner to read your iris right. This is obviously understandable considering the camera resolution and the details needed to make this feature working in a consistent and secure manner. As for now, we don't really have any better option than the iris scanner and the fingerprint sensor. Having said that, the phone supports a wide variety of unlocking methods, some are secure, others are much less. This is why I personally just put the 4-digit pass code and never unlocked my phone using the fingerprint scanner, it's just very uncomfortable to use on my iPhone.
Here is an excellent video by Tim Schofield YouTube user, which demonstrate the new iris scanner and how it work in practice.
Is the Iris scanner useless? I don't think so. I think that some people will find it better to unlock the phone using the iris reader than the rear fingerprint sensor at the back of the S8.