I think it's a little harsh to say the movie was "nothing more than a brilliant marketing tactic." I think all things considered, it's an excellent horror movie. Yes, there was a backlash to the hype, and it didn't help to have Time Magazine declare it as the "the scariest movie ever." But there are some genuinely scary moments in that film that hold up especially well, and the ending in the house is one of the most unsettling finales in modern horror (so glad they didn't change it!).
Also, having been a Blair Witch junkie and recalling that lead-up and launch of that film quite vividly, I'm not sure I agree that "countless people" were fooled into thinking the footage was real. I think a lot of people, including myself, were intrigued by the marketing of the film prior to the release and wondered if it was a documentary or not, but I don't think all that many people went into theater thinking it was real, to say nothing of coming out of the threater. I could be wrong, of course.