I'm largely with Porter here.
And I think signs and such done in Comic Sans do look amateurish, but I think that typically either they are intended to look this way, or they simply are done by amateurs.
As far as readability, Comis Sans is one of the more readable typefaces out there. I think the vast majority of typefaces that people get all snobby about are rather unreadable. I think all of your basic needs are met by a nice serif font like Times Roman and a nice sans serif like Helvetica/Ariel. Anything else ought to be reserved for titles or not used at all, pretty much, but I consider Comic Sans to be an exception, in that, unlike most of those fifty thousand fonts that come with most software these days, it actually is pretty readable. (Another exception is Courier, in case for some reason you want something to look like a typewriter typeface--though in general I find Courier hideous.) I certainly would not want to read a paper written in Comic Sans, but for a little paper sign or personal web page or AIM font, I see nothing at all wrong with it.
Most people who complain about Comic Sans are not referring to "professional" products or thinking of the poor graphic designers driven out of business. Mostly they just make fun of people's personal web pages (or those of small businesses) or their AIM fonts, and I can't distinguish between that and being a snob.
-Icarus