Hi virg - welcome to the site.
One reason I've not invested a lot of time looking into HHO generators is because they're so closely associated with and marketed like other MPG scams out there. That alone, to me, is a huge red flag. (You have no idea how many "run your car on water!" ads I've had to exclude via the Google Adsense filter).
I tend to agree with the positions taken by the writer of fuelsaving.info in general, so I rely on the points on his page advising skepticism about bolt-on HHO generators:
http://fuelsaving.info/hydrogen.htm