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Pro Independent of Google services
Almost all Google services are removed from this Android-based UI in favor of creating a competing ecosystem of apps, games, themes and services.
China government prohibit the Google services
Pro Combines best of iOS and Android launchers
MIUI has visual similarities to iOS and TouchWiz and combines functionality from both iOS and Android OS and its variants. For example, MIUI has no apps drawer and instead puts all app on homescreen pages the same way iOS does.
Pro Runs faster than stock Android
MIUI includes performance tweaks that give it a edge in speed compared to stock Android.
Pro Detailed installation instructions for supported devices
Step by step instructions (with screenshots) for flashing MIUI ROM can be found for each of the supported devices on the companies website. For example, here is the flashing guide for Nexus 5.
Pro Fast development cycle
New features and updates are released regularly.
Con Some bundled services are available only in Chinese
Xiaomi, the developers of MIUI are Chinese and their primary maket is China. As such primary language for most apps and services is Chinese and not all of them have yet been translated to English or any other language.
Con A Mi account is required for using bundled services
Various services and apps require a Mi account. For example, buying new themes requires spending Mi currency that can only be obtained if a Mi account is available.