as for "normal", anyone has it's own measure. Pentax believed in 42mm and made "normal" limited lens in this FL. Actually when I have to go with one camera, 40mm is good "in-between" from 35mm and 45mm. So I believe in small 40mm lens....or small 25-50 zoom, if I feel 40mm will be too restricting.
Back to attachments - some people demonstrate good pictures with them - quality is also relative term. I think they worked in past when you had to shell out lot of money for gear (to go wider or more tele with SLR), but this days it's just curiosity which gives use to this attachments.