Depends on the person. The issues vary widely. Many have no issues, for me, Cortana was broken if I tried to use speech or in various other situations over the builds, over hard resets, etc.
Its usable, but depends also on how much of a stickler you are for quirks. I am a big one so that's half the reason I don't use it. (and also a quarter the reason I dont use webOS right now.)
however, do expect mediocre to poor performance/reliability with Facebook and some other apps. W10M is still less stable than its competitors as well as 8.1. But you do get some features.