I was diagnosed over a decade ago at the age of 19 and have been in treatment ever since. The changes as a result of getting medication and attending therapy have be profound and decidedly mixed. One thing I'm just going to say upfront is that if anyone reading this thread decides to seek treatment, please don't solely rely on medication. The medication allows your ADHD to become manageable, but crucially it does not manage it for you.
Good changes:
1) Social life immediately and greatly improved. I was able to hold my attention when talking to people. My tangents became less wandering and frequent. I had the focus to make plans and actually follow through with them. I texted people back (this alone was huge).
2) I lose things less often. Before starting treatment I was notorious among my family and peer group for losing things. I still lose things all the time, but now it's usually because I put something down in my house rather than losing things in public.
3) Improved physical health. The appetite suppressant aspect of stimulants play a role here, but I think the bigger thing here is less impulsive food choices. Because I just don't feel as hungry, I have to plan my meals better and so I eat healthier. It's also easier to stick to an exercise schedule.
4) I can sit still and be quiet. Looking back at my childhood, it should have been fairly obvious what was going on, but I believe the mindset was medication was only for kids with poor academic achievement. The scale of lost time has been personally frustrating, but I am grateful for not being put on medication when I was young child.
Bad Changes:
1) Massively increased libido. The stimulants can increase sex drive, but can also make orgasm more difficult while also sometimes making it more difficult to maintain an erection. While these last two can be frustrating on a personal level, the hyper sexuality can be quite destructive when not kept in check.
2) The hyper focus aspects of ADHD are now basically on all the time. Medication is less a guided missile and more a laser. Instead of taking it and 'magically' getting things done, your attention is now dialed into whatever you point it at. This can be extremely productive, or not. I have less 'lost days' than I did before seeking treatment but those days now have a greater intensity.
3) It can be easy to get into stimulant fueled cycles of self-destruction. I need to be very disciplined about getting enough food and sleep. The drugs make it easy to shrug off a night or two of little sleep. But using drugs to ignore my body leads to much worse ADHD symptoms then if I hadn't been taking the drugs at all.
4) There is a distinct difference in my personality on days when I take my drugs and days I do not; see Good Changes, point (1). This can be hard for other people, especially romantic partners, to deal with.
Overall I'm happy that I've pursued medication in combination with therapy. It has let me get a handle on my life, but anyone who says it's all good is not telling the truth.