Generally, I'm really impressed by Face ID. I would say it works more reliably than Touch ID for me in day to day use (my last phone was a 6s so not the latest gen sensor), and haven't really had any problems unlocking it in bed etc. The ability to glance at the phone to preview the notifications feels really futuristic. The one issue I have noticed is that really strong sunlight seems to cause issues, I guess because there's too much light. I've only had this once, on a really sunny winters day by the seaside.
Couple of things I'd like to change about it:
1. It would be great to be able to only show notifications when you are looking at the phone, even when it is unlocked. Currently, if the phone is unlocked and notifications are set up to show when unlocked, it will always show the contents of a message regardless of who is looking at it, which almost caused a slightly awkward situation recently when I handed someone else my phone to look at photos – I was under the impression notifications would hide the preview if I wasn't actively looking at the phone. This makes sense, but it would be great to have the option. Of course, there are probably reasons why this could be problematic, e.g. battery life.
2. Face ID isn't natural to use when your phone is flat on a desk. You either have to lean over awkwardly, or tilt the phone up, whereas with Touch ID you could just touch to unlock. Incidentally, a back fingerprint scanner wouldn't solve this use case, so I think Apple might the right decision by focussing purely on Face ID and making it as a good as possible. I was wondering whether a wider angle front lens could solve this issue - if you look at the selfie camera output, it wouldn't have to be that much wider angle to capture your face in most "on desk" scenarios. Of course, this might have the knock on of making selfies look less attractive, so maybe it would call for a second camera lens just for Face ID?
It's not a huge deal, as when I am developing apps (which requires frequent unlocking) I have the phone tilted so it can see me, so I can just glance at it to unlock when launching a new build from Xcode (which feels very futuristic!), but it would be nice to have a solution, and I can't imagine Apple adding Touch ID back in even when under-screen fingerprint readers become a thing – it would feel like admitting Face ID wasn't perfect, which isn't very Apple!