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Just installed it yesterday. It works just fine. You might see a hiccupnevery now and then. Something around iOS7 in a 4S standards so still fast regardless.

Running the latest OS on a 70$ phone (sometimes 50$ if you know where to look) simply cannot be beaten.
 
"Works great" is relative. The performance of 8.1 on my 520 is excellent. But since 8.1 is a work in progress some things don't quite work well, if at all. Syncing with Google services (mail/contacts/calendar) are sporadic. It works for some people, not for others (including myself).

There have been changes to the music player in 8.1. It appears to mangle metadata tags for certain mp3 files. I'm trying to narrow down the scenarios where this is happening.
 
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