My client wants to consider a “walk on video” where it appears as if a person is talking/walking onto the web page. There are many services to do this including actors. I don’t have data to support my bias against doing it. I imagine if it wasn’t effective, site owners would not continue … but it must depend on the nature of the site and content…yes?
I guess people must consider them useful, as they are common. But I find them tacky, and am trying to dissuade a client from using one right now. Instead, I personally prefer a traditional video near the top of the home page (rectangle, fixed in place like an image). A lot of sites have a nicely styled intro video, and it can be done very tastefully. Unfortunately, marketing seems to have little to do with tastefulness. 
Definitely seems like it would have to fit with the overall aesthetic of your site. Although some of these can be engaging, there might be a point in the near future where it becomes more of a nuisance then anything else (i.e. “oh, one of THESE things again”).
I think many people do better with videos as seeing is a lot easier than actually reading. I know I personally learn a lot better when I actually see something!
With the rise in popularity of video sites like YouTube, I do believe that Walk-on Videos are very effective. Combined with website search engine optimization, a dedicated YouTube channel created specifically to brand your dealership, promote cross-selling of your inventory and manage your videos there’s no doubt that your ROI will increase.