I switched from an iPhone SE to a Xiaomi Mi A1 last month. It’s an excellent device for the price (under £180).
The hardware downsides are
no water resistance
no NFC
no Qi charging
none of which are particularly important to me.
The only software downside is that the stock kernel doesn’t come with camera2api or electronic image stabilisation enabled. These can be enabled fairly easily and, with a modded version of the Google Camera app, dramatically improved photo quality can be achieved. I’m hopeful that Xiaomi will enable these features in the Android 8.1 update, or Android P.
I’m a big fan of Xiaomi’s stuff, which tends to be good quality at reasonable prices. I’ve had their earphones, Mi Band 1S, Mi Band 2, Bluetooth gaming controller, smart scales, and now the Mi A1. I also considered their Mi Book Air laptop, but am holding out for a Ryzen-powered version.
Actually Xiaomi phones have been quite popular back home, and my brother has been a fan and owned a few. I think last half a year there has been a huge uptake as previously those phones were only shipped through re-sellers like GearBest but nowadays more and more operators offer them directly and I think some high-street retailers too so they are getting some exposure over here.