I had a bad case of teeth crowding. I had three teeth removed to make space and that was the most painful part of the whole process.
When i put new trays in, it hurts for one to three days (today it is one of those days), especially when I have to put back the trays after eating. Ibuprofen really helps with the pain. Even Advil it's enough sometimes. In some cases the change was almost pain free, so i didn't had to use any drugs to alleviate it.
Currently, I am at my 20th of 38 upper tray and 16th of 34 lower tray and I'm very happy with the results so far. I didn't entirely followed the recommendations about drinking only water with the trays on, but i didn't drink hot liquids with them on. Also, with proper oral hygiene (at least 3 times a day brushing and flossing at least once a day), i didn't notice any staining caused by drinking other beverages then water.
I don't know if everybody will get the same results as I did, but considering my results, I would definitely recommend it.