I'd anticipated starting HRT for decades. When I finally pulled the trigger, I felt euphoric at the thought that I was fulfilling a dream.
In the buildup to starting, I'd read about "brain fog" clearing, seeing the world in multicolor and all that sort of stuff. I'd also read that I'd know it quickly if it wasn't right for me.
None of that happened.
Instead, I just experienced that low-level euphoria over switching hormones from male to female. And my body started to change. Breasts were first and the most pronounced. I'd say that the second was my butt - my underwear started to get tighter back there. Somewhere along the way (several months in) I noticed that my skin got much softer. I bruised more easily. Body hair started to disappear. As others mentioned, these changes kind of crept up on me. I loved every change, which helped to affirm the decision that I'd done the right thing.
I should point out that I started at a low dose, stopped at times, then a higher dose, then lower, then higher. I believe that positively impacted breast development, as early maturation is said to lead to a cessation in growth. But it's quite possible that the mental changes could have been more profound on a higher dose. These days, on a full dose, I can feel mood changes that sometimes relate to changing levels. As I understand it, it's not really female hormones that cause mood changes, it's fluctuating hormone levels. That's something unexpected that has taken me some time to get used to.