95% of the apps are slower than before, system is slower, not major slowing though
and some of the apps need reinstall to make it work again, some apps are gone (here & lumia etc, and the new ones are supposed to do what here and lumia did in the past but the new ones are just buggy currently)
The store is still kinda problematic, random errors, even reinstall can have error....(due too unsuccessful deletion even the storage/app list says otherwise, usually a reboot/ temp memory deletion will fix it)
new native/preload app is lacking basic functions that we used to have in 8.1, like sorting in onedrive, team/sport pinning, public transit for direction in map (depends on country), outlook mail is not working right in a lot of ways (not syncing not displaying, no new mail count [wtf])
disclaimer: i did make it sound bad, but thats what we do, we mention all the bads and take the goods for granted. NONE of them are deal breaker to me, but if you live on emails, you will die if you missed one, then i suggest you use web version lol. I can still say this 10166 build is solid, but can do better. oh and i am using 1520