I love this GRAV grinder because it doesn't have a screen, so you're not filtering out all the "good stuff". I find i just have to put a little bit in and it grinds everything perfectly. I feel like a lot of people overpack the grinder and it gets stuck - a little is all you need! It has a nice metallic surface that feels sturdy and good in your hand. Overall, this is one of the best grinders I've ever had.
my experience with 420 science has been great. no complaints there. this grinder kinda sucks though. the teeth just don't really do anything. you put the herb in, twist, and it's stuck. doesn't shred without using a ton of force, which inadvertently tightens the lower chamber and makes it really hard to get apart. I have to break my herb into tiny tiny pieces, basically ore grinding the herb before I can grind it. my other grinders have no issues with the same herb