I have very sparse eyebrows - I think hormone related - and I looked at both Revitalash and Rapidlash - but the brow versions not the lash serums. They both got good reviews but Rapidlash is half the price of Revitalash so I decided to try that one first.
You have to apply it morning and night and they warn it can take up to 8 weeks to see a difference. I actually found that I could see a difference after around a month. It certainly wasn't a miracle but the brows I did have grew longer and there were more hairs visible at any one time, I think this is due to the way the hair cycle works. However, where there wasn't hair to begin with it didn't make it grow back.
The effect is only temporary too - I went on holiday for two weeks and forgot to take it with me and found the results were really almost gone completely after two weeks of not using it.
It is quite tricky to fit it in your routine too as you're meant to use it on clean dry skin. My skin is very dry so I use a lot of spritzes and hydration products during my morning skin routine so it was difficult to know where to use it. If I put it on first then left it to dry before applying anything else and risk diluting it the rest of my face would dry up really badly. I ended up applying everything else first and trying to avoid my eyebrow area then putting it on last and then waiting for it to dry before using spf. Evening was easier as I usually do my routine a bit before bed so everything else was dry before I applied it.
Overall, it was definitely effective and there was a noticeable improvement, but only where the hairs were actually still present and if you were completely consistent with using it as per the instructions.
I'm going to splash out on the Revitalash brow product at some point to see if that works any better on the areas where the hair growth has stopped altogether.