For a solid year I didn't leave the house without Toppik in my hair. It was a step, same as mascara. Dry shampoo, Toppik, check the parting, done.
What's the difference between hair fibres and scalp concealers?
Hair fibres are tiny fibres you shake or pat onto your scalp, and they cling to the hair you've already got so thinning areas look denser. Scalp concealers tint the scalp itself instead. A lot of what you see when hair thins is pale scalp showing through, so cutting that contrast makes your hair look denser without adding any coverage at all.
Different jobs. Both useful, and they're a good team together.
Do they grow hair back?
No. Neither of them treats anything. They're purely visual, and that's fine, it's what they're for.
The downsides
Rain can shift fibres, even light drizzle, and a strong gust of wind can send a little cloud of them off your head (startling the first time). Hairspray helps set them, but they're not waterproof.
Don't touch your hair once they're in. Brown fingers, stained nails, the lot.
Colour matching can be fiddly if your hair is very light or an unusual shade. Going slightly darker usually looks more natural than going lighter.
The ones I've actually tried, with the short version up top, are in my best hair fibres UK post.