I don't like my options on this poll. There needs to be an "all of the above" option. I've used my steam cleaner for engines, interior, and exterior in one way shape of form.
I really get use out of it in the winter. Cause around here in NH, you can't really pressure wash an engine in freezing weather. So I take care of it with the steam cleaner inside.
I also can't do as thorough of a wash as I'd like. So I use the steam to get the stubborn areas more to my liking. I also find it handy to loosen stubborn stains before extracting. If I get something bad I'll spray some degreaser, then blast it with the steam to loosen it up. AND another one of NHs great gifts...road salt. Works great to break up crystalized road salt in the carpet. Uses are endless. So, I choose "all of the above", haha.