Curse is something I started using more as the beta went on because I started to realize its potential.
I think Curse is pretty useful. Though it’s harder to hit on smaller enemies who move around a lot, it’s really rewarding if you do hit it. However, Thorns abilities and uncharged arrows still do enough damage to them anyways. Curse really just adds a lot more burst to her abilities, which is awesome. She’s already got high burst, especially at level 5. Add a 25% (35% at level 5, if you take it) bonus to spells and charged arrows, and you’re looking at some high numbers.
A character like Thorn should have a problem with tanks, but instead she shreds them because of curse. If you curse a tank (or super minion) and hit them with every volley arrow, you just dealt a lot of damage. Then if you took Piercing Volley and shot at their feet so it bounce up and hit them twice, you just did EVEN MORE.
I think it’s kinda meh that uncharged attacks don’t get a buff from curse, but that might make it a tad strong.