I just ordered a sine // saw, Firmy, and a shinwoo Dolphin.
Im looking for opinions an the sine // saw mostly but others are OK
The Firmy is very fast, aggressive and a V shape. I really enjoy this one a lot. Not to big, not too small. Not a great grinder if I recall due to the smooth finish. Budget in price, amazing on performance. It’s the most affordable sOMEThING that’s metal.
The Sine//saw is fast, not nearly as fast as the Firmy due to the shape. Depending on the finish, it may or may not be a good grinder. I know the laser etching on my red one helps with the grinds in that first ring area after the rim. The Violet/Gold Paisley one I just got is a much better grinder as the surface appears to be soda blastedComfortable, still a V-type shape, but way more rounded, which makes it more comfortable. Another serious performer. The only downside is the STAX axle system makes this yoyo not so ideal for gyro tricks because the raised nub is in the way, but otherwise, horizontal play isn’t a big deal. Like other RecRevs, a tremendous value.
Sine//Saw rocks. Best 65$ ever spent on a yoyo in my case. It’s just SO GOOD. Just as good as the PV featuring Anthony Rojas 
The sine//saw is really amazing. Really fast and stable, dead smooth, and great for horizontal. It is actually so good, that i now can only throw Rec Rev. In my opinion, it out throws any of my other yoyos.