Can the X really replace a camera? I thought cell phone cameras are always going to be lower in quality than a digital camera.
It depends on what camera you are comparing the iPhone X to, and what type of photograph. I've been shooting with Nikon DSLR cameras since 2004, and these days I shoot mostly with an Olympus OM-D m4/3 camera (also interchangeable lenses, but smaller sensor than my Nikon). There is another very long thread on this topic that has been peaking over the last 48 hours. Here's a summary of my opinion
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No single camera+lens combo is great for every type of photo, so a lot of it comes down to what you like to photograph, what the conditions are, and what your expectations are. Interchangeable lens cameras exist to provide flexibility and maximum quality. Superzooms exist, but they have their compromises in terms of lens quality. For general photography and snapshots in good light, typically shared on social media, the iPhone X will compare quite well to any camera covering a similar field of view. It may not always have the pop and detail of shots taken with more expensive cameras and lenses, but results will look great for a lot of shots. A smartphone also won't be as nice to shoot with as a camera with a viewfinder and manual controls, but it will be easier to carry. This is why the smartphone has pretty much already killed the compact point-and-shoot camera market. Smartphone cameras reached the "good enough" level for the general public a few years ago. At that point they were already better cameras than the 120mm film compacts and disc film cameras that were so popular in the 1980's and 90's.
DSLRs and other interchangeable lens cameras (and some high-end compact cameras with built in lenses) have larger sensors (not just higher resolution, but physically larger) that perform better in lower light and also scale up to large prints better than smaller sensors producing similar resolution. They can also pair with high-end lenses designed to deliver maximum detail and contrast. Larger sensors with longer focal length lenses also allow you to control depth of field more... blurring the background when you want to. Small sensors with short focal lengths, like you find in smartphones, have enormous depth of field... everything is in focus even when the telephoto lens is used. This is why current flagship smartphones use software to blend exposures and blur the background. Of course that doesn't always end up looking as natural as when it is done using optics, but dot projector technology is changing that and the software will keep getting better.
So in short the X can't really replace a high-end compact camera or an interchangeable lens camera with a bag full of lenses for all types of shots, but for some types of shots you'd be hard-pressed to see the differences. What the X can easily replace is a small-sensor compact point & shoot camera that has a 3x zoom and no viewfinder.
Sean