I went to a midnight showing, full of adults and teens, all of whom were clearly Indy fans. I was in costume, as were several others.
There was definitely an atmosphere of excitement, and when the lights went from "semi-dim," as they are during the previews, to "totally dark," there were several cheers and hollers.
The other biggest cheering moments were when Indy's silhouette put the hat on, when Marion was brought out of the tent, and when Indy snatched the hat out of Mutt's hand at the end. There was also a good amount of laughter at the appropriate moments.
I honestly can't remember if the audience clapped at the end, but people were generally happy with the movie, based on the comments I heard. For myself, my reaction then (as now) was "Not as good as the others, but a pretty good movie."