There is evidence starting to surface from animal studies that show potential benefits and NIDA has sponsored a 5 year human study with the University of Florida Pharmacology Department which is very exciting!
All in moderation! If you have to increase the dose, take a break and let your body recharge what the Kratom is "supplementing". There is no true answer to what Kratom does in long term, like what Philip R said.
The exact effect may vary from person to person just like it does with medications. Some people will respond in one way, some in another. It's sort of like coffee; some people swear by one blend and someone else may prefer another.
I think it varies from person to person. I don't think there's any bad effects, I have never noticed any myself. Long term use may vary. Use in moderation, you should be fine.
Thanks! Your answer is awaiting moderation.