My Lumia 520 works better then before! I love the app installation onto SD card and extra tiles, notification and Cortana is amazing! For some it has been slow, for me, WP8 was slow, and WP8.1 is sooooo much faster, and I have way less 'resuming' or 'loading' time.
Same here with a 521.
The Lumia 521 with 8.1 is like a device reborn IMO.
Far as bugs there's is the normal complaining about growing pains.
However things like:
Storage Sense
start+theme
notification+actions
Wi-Fi Sense
Word Flow
Separate volume for ringer and media
The new WP appstore
Back-up and phone reset
...and of course Cortana (US)
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All the above in my opinion have definitely pushed the 521 further up the ladder.
I just did a phone reset last evening to see what would happen.
Everything on the phone came back except the 3rd row of tiles and apps I left on the SD card.
Being able to move\install apps to SD has freed a lot of internal phone space.
The 521 user with a large fast SD card no longer has to worry about collecting store apps.
Out of 7.2 gig available I'm showing 2.5 gig used 1.9 is system plus whatever else.
4.7 gig free!
The notifications pull down is pretty cool also.
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Had nothing but the best experience with the 8.1 preview on the 521.
For me and the 521 it has been what I would call a galactic improvement.