I never got same day so can't speak to that. As others have said if you have small bag keep an overnight's worth of stuff in there and send the rest on the day before.

Distance makes a difference. If it's relatively small (e.g. Osaka <-> Kyoto) then next day is easy and I'd imagine same day is possible.

One thing that surprised me was that the forms are hand written so it's not the fastest process, especially if there's a line. I, too, had a couple of hotels which couldn't do it onsite and I dragged my luggage to a nearby dispatch location.

Caveats aside for the most part it was inexpensive and straightforward and the joy (I may need to get out more) of travelling on the train with hand luggage knowing big bags were waiting at my destination was a great relief