There is almost never an excuse for using Comic Sans. If you don't know what the story of Comic Sans is, I'll inform you that it was designed by Vincent Connare in 1994 in an attempt to make a better typeface for interface purposes, specifically speech bubbles, than the Times New Roman that was used by Microsoft until then. Unfortunately, it's infected the world since by being included in the standard font list in Windows 95.
Comic Sans is a detestable typeface. Despite being ostensibly based on the lettering of such graphic novels as Watchmen, the typeface has none of the style of these hand-drawn letters and instead is infested with inconsistency. It is a disgrace to the world of typography. Connare might have later made up for some of his sins with Trebuchet, which isn't terrible, but his most (in)famous is still free to roam around, destroying any semblance of good typographical design in its disciples.
There is only one positive thing which can be said for it: At least it's better than Papyrus. There is almost never an excuse for using Comic Sans; there is
absolutely never an excuse for using Papyrus. In all other respects, Comic Sans should be considered a blight on the typographic universe. I think I'll stick to my Avenir and my Adobe Garamond, and if I ever feel like conveying a feeling of irreverence, I'll choose a rounded Helvetica or something.
I leave you with this, an ostensibly serious political letter that I received recently:
Comic Sans and Bradley Hand: If these two typefaces being used for what is supposed to be a serious political letter is any representation of Gay Mitchell's campaign, making him the President would be a joke - and not a particularly funny one.