I strongly recommend this ...

I strongly recommend this from birth until your child is old enough for for their first toothbrusht. I will also be recommending this (even if I have to buy it) for any future grandkids too.
I used the older version of this when my 33 yr old was little. I also used a slightly newer version of this when my 24 yr old was little along with an older version of Orajel Toddler Training Toothpaste. And for my 8 yr old daughter when she was just a little one, a still newer version of this silicone finger toothbrush & gum massager, again with Orajel Toddler Training Toothpaste.
I then used these, until she was old enough for her first toothbrush. I works great, and I had no trouble with this product over the years.

Works as intended, but not for larger fingers

Can't comment on this as a baby toothbrush as I have bought it to use as a cat toothbrush (great for tricky kitties who don't like brushes but will let you rub their teeth; make sure to use special cat toothpaste). As a toothbrush, it works really well on cats, thick enough to stop you getting poked by them sharp teeth.

The downside is this is very small. I am plus size, but my fingers are on the thinner side for women my size (still bigger than average though) and this fits...just. The problem is when you try to take it off, the pulling causes suction on the finger and it's almost like a finger trap. It should have an airhole in the top to prevent this. Thankfully it's flexible enough that you can roll it off, but it's a bit tight.

If you have hands on the large side of average or are a man, you probably will need to put this on a smaller finger or find a bigger option. Still, it works well, just not if you have large fingers.