If you want a small-screened but powerful phone in the premium space, your choices are massively restricted, with Sony more or less just taking on the Apple iPhone 8 in this regard. Just about everyone else's smaller phones are running on older, less powerful hardware.
The iPhone 8 is a nice phone, and if you're in the iOS camp, it's the phone to buy. However, the Xperia XZ2 Compact is in just about every way a better device overall. It's not just the "default" winner of the small powerful phone race, but a good phone in its own right, even if it is within a rather pudgy case.
There is a challenge in pure value terms here, and it's one that you should consider before buying the Xperia XZ2 Compact. At $949, the Xperia XZ2 Compact is a little less expensive than comparable handsets, but you've got to really love that small form factor. Jumping up around $100 or so (or less on sale), and you'll get a lot more screen space to play with.

Alternatives
Unless you really haven't been paying attention, if you want a small and powerful handset, you're bereft of real alternative 2018 choices within the Android ecosystem. If you don't mind jumping over to iOS, you can consider the Apple iPhone 8, but be prepared to pay more for it.
Within the Sony family, the Xperia XZ2 is clearly the closest match to the Xperia XZ2 Compact, but it has issues around its rather slippery design. Frankly, if that appeals to you, you'd be better off with HTC's slightly nicer and similarly priced HTC U12+. If you're after a more richly featured camera phone at around the same price, consider the Huawei P20 Pro, one of my current favourite flagships.
If you're keen on a Snapdragon 845 phone and you can find one, you could alternatively opt for the impressive OnePlus 6 handset, which you should be able to find at around the same price point as the Xperia XZ2 Compact or possibly even lower.

What the other reviewers say
Site | Comment | Score |
---|
TechRadar | "Small and powerful but not very stylish."
| 4/5 |
Engadget | "It's by far the best small phone money can buy." | 90/100 |
Tom's Guide | "Once in a blue moon, a product comes around that inexplicably dazzles me. Sony's Xperia XZ2 Compact is one such device." | 8/10 |
Trusted Reviews | "If you want a small Android phone then the Sony Xperia XZ2 Compact is the best choice." | 3.5/5 |
Android Authority | "The Xperia XZ2 Compact is expensive and unless you highly value its compact nature, it isn't worth the purchase." | 8.8/10 |