Absolutely enjoyable for what it is and for what Castle wanted it to be. It's a gimmick through and through, and that's precisely its charm. The loveliest part of it is that Vincent Price is so perfect a movie star (one of the most perfect there ever was) that he plays it in full seriousness.
I kind of feel cheated that I didn't see this in a movie theater, but on the other hand, is it a good thing for a film to rely so much on interactivity? Castle's schlock horror does work on its own, but it's also quite slow in getting into the point. The bathroom scene does elevate this above many others, though, along with the presence of Vincent Price (seriously, what film would he honestly drag down?).
Not only is the plot ridiculous in a non-fun way, the whole "Tingler is in the theater" stuff is absurd. If the idea is the audience is supposed to be representative of the real audience, why in the world would anyone cut the lights (twice!) when a creature is there? You would turn ON the lights. Also, I got pretty sick of hearing the word "tingler" 7,000 times.