General

Yes it is worth it.
I used to have 25 att on mine.

I wouldnt say so, pretty $-hitty link skill, rarely ever over 5

They changed it so it will always give you 3 atk +3 all stats. There is no way anymore for me to change my core, and double clicking stardusts does absolutely nothing.

at the very least, you'll get +3 all +3(m)atk.
sometimes, you'll get a pretty good boost, like +10+ attack and/or stats. Last week, I had +15 attack. The previous week, I had +14 attack with +18 luk, or something like that. I don't buy NX, so these stats were from the regular stardust that drops from monsters.

Not worth it if you're only trying to obtain the link skill. Most days, you'll only get +2 m.att/w.att and +2~+5 of your main stat. But there are those days where it becomes +10

Mine randomly reset to +18 int and +15 m atk, but I did need 1.5k nx to lock it for a month. (for my F/P) so it suited, but it could of had the "wrong" stats. Seems worth it to me.

It's okay, you may need NX to get the best stats though. But it is also possible to get +40 Stats and +40 M/ATK or ATK