I too, use the file, in the field and at home. I used to take them to the local mower shop and they do a decent job, but at $7 a shot the price of a chain goes up considerably. Since it's 25 miles to town, I could only get them sharpened when I needed something else. That and they NEVER did it same day. That was frustrating, so I got my kit from Stihl and never looked back. They have great videos online and after watching a few, I have saved money and time really.
And like others, all I was getting was about 4 sharpenings per chain.
If I am going on a big cut, I always take extra chains with me. You never know what you might hit. Once helping a brother in law cut up a fallen oak out of his horse coral, I noticed the saw wasn't cutting well at all. Looked all around the log and it had fallen on a fence "T" post. The post had a nice shiny groove in it. Totally ruined that chain. Duh!