Some things are a matter of opinion. Some things are a matter of fact. This is fact: Comic Sans is evil incarnate. I am not unique or original in this view, which as mentioned isn't actually a view but a fact, but it is worth pointing out here because for some reason, not everyone is aware that Comic Sans is evil incarnate.
Comic Sans is a font. A typeface. A Microsoft standard font released in 1994 to ape comic book lettering for kiddie stuff. For no reason at all, the world went entirely mad when they saw it and fully-grown, seemingly professional adults started using it for formal communications. At work. For business. This includes businesses that involve children. Even though the communication is directed at the grown ups.
This forced what was a perfectly fine cute and wacky' font to become the most hated thing in design since clipart, and rightly so. There are situations where it is simply not appropriate to communicate using a font whose personality is childish, immature, and incapable of making robust, rational decisions. If that is the personality you are trying to imbue into your writing, then go ahead and use Comic Sans. Just bear in mind that using Comic Sans gives exactly the same messages as! using! an! exclamation! mark! after! every! word!!!
For those of you not familiar with it, you might not think that such an innocent-looking font could possibly give off that much "this communication is FUN!" evilness, but you must bear in mind that the font is now so well-known, it labours not only by its 'cute and wacky' looks, but under the weight of its reputation. Here are some web resources expressing the level of hatred for Comic Sans:
Ban Comic Sans website.
A web comic.
A message from some cartoons.
Loads of great examples of inappropriate Comic Sans use on Flickr if you search, but this is my favourite.
And if you are not sure when it is appropriate to use Comic Sans, here’s a handy flow chart.
The 'sans' in Comic Sans refers to it being a 'sans serif' font; that is, without extra decorative lines like Times New Roman etc. 'Sans' simply means 'without'. So a literal definition of Comic Sans could simply be NOT FUNNY.
I concede that blaming the inventor of Comic Sans for its misuse is like blaming Einstein for nuclear bombs. Remember kids: Comic Sans does not annoy people; people annoy people.
On the other hand, he created a monster. He must die. Or at least give a hundred dollars to everyone who has to look at their work Christmas party invite, created by the token office compulsory fun organiser, which has Comic Sans, clip art, PowerPoint 'word art' styles, and plays a tune and opens a broadband-bandwidth-depleting You Tube video without your prompt.