My experience with it is that you will indeed regrow at least a little hair, with enough time.
Once you drop down to female levels of testosterone, further hair loss will stop, and you'll keep everything that's already there.
Hair follicles that were weakened or rendered inactive by DHT will indeed recover, and slowly start regrowing, with time. I've heard sources say that the most HRT can regrow is about 5-7 years worth of hair loss, and it takes about 3-5 years for the full effect. So it depends how much you've lost, and how recently. And again, it can only recover follicles that have been weakened, not killed off. So basically, if an area is shiny, the hair is dead, it's not going to come back. If there's still little blond peach-fuzzy hairs there, it's only been weakened, so it will slowly start recovering.
I started HRT with a receding hairline and very thin hair on top of my head. After about 3 months on HRT, I could see peach fuzz growing in droves on the corners of my hairline. And slowly, slowly, slowly, ever since then, it's been turning back into dark active-follicle hair again, little by little, starting at the hairline and slowly filling in.
It's not a lot. Admittedly there's still more peach fuzz than actual hair, with only the very edges filling in, so I still have a LOT of hairline recession after 18 months on hormones. And I'm not going to get back the full completely-thick head of hair that I had pre-puberty. Even at age 20 I was already thinning very visibly, and I'm 28 now, so that's probably the best it's going to get again. But the peach fuzz continues to grow more and more with each passing month, so I don't know, hopefully it will keep going. My hair still looks extra frizzy because there's all of these little short hairs still popping back up, and they keep coming and coming, so who knows.
Again, it takes a LONG time. 3-5 years for it to reach its full recovery potential. And it's not a lot. It's just enough that people have commented me by saying "wow, your hair has really been getting thicker back there" when looking at the back of my head, but not enough to fix it completely.