It's been 15+ years for me (although with lots of on and off), but if you're used to living with ADHD, it can feel like a fog is being lifted. You can suddenly hold more in your head at once and don't have to mentally "drop" a thought to be able to process other things. It's like slotting in an extra stick of RAM.

The meds probably have that effect on anyone, but it's profound if you've been working with a deficit.