This is currently tied with Falsies (waterproof) for my favourite mascara. The brush is great, because it's fine-bristled and narrow, so I can get mascara get every lash in a way that I just can't with the huge brushes other brands are using.
Stiletto is a much more flexible, versatile product than Falsies. It's a wetter formula, which means smoother application much less prone to any clumping. I haven't needed a lash comb or separating brush AT ALL with Stiletto -- it's as if the lashes just spring apart, and because the product takes a little while to dry, additional coats don't grab on in a chunky way.
As a trade-off, it's not as volumizing... but you can apply many, MANY (many!!) coats smoothly. It never gets to insanely dramatic levels of volume, but it looks amazing even close up, unlike most volumizing formulas. So it just depends on the look I want.
On using the S-curved brush: I start out with the concave portion on my inner & middle lashes, to encourage curl in the centre. Then I flip it over so that the outside lashes get a dose of concave brush, which keeps them from drooping. Alternating like that is pretty much second nature when you're applying multiple coats (which I always do), so it's fine. You'll see what works best for your eye shape & the look you prefer.