In every line of thinking there is always a great debate that will never be satiated. Creation vs Evolution; Nature vs Nurture; Britney vs Christina; Butter Side Up vs Down. When it comes to design the debate is almost always Readable vs Artistic. Is it more important for your logo to grab someone's attention or convey what you do? Is it more important for your site to look cool or be easy to navigate? It it more important for your ad to be legible or to look awesome?
LifeHacker has a recent post claiming that "Hard to read fonts can make text easier to remember." They site an example in which 200+ grade school students are given a story in different fonts. The facts given in hard-to-read fonts were retained more often and with more clarity than the facts in easy-to-read fonts. The implication here is that if your ad is difficult to read than your ad is memorable. If your ad is easy to read than your ad is forgettable. There are obviously a couple of problems with this suggestion.
The study here is acting under the presumption that Comic Sans is what passes as a hard-to-read font. Let me be clear on this. Comic Sans got it's name because it looks like a font you would find in a comic book... for children... in small print. Comic Sans is not difficult to read. It's not distressed, destroyed or distorted. It is crisp, light and energetic. One might say it has excellent perspicuity.
The study here is acting under the impression that a grade school survey is a practical medium of advertising. I once took a 48 page survey in college because I had to. My grade depended on it. Unfortunately no one has anything riding on your advertising... except for you. No one has to read your ad! My chief concern with hard-to-read fonts is NOT that they won't be memorable. In fact, it would stand to reason that they would be. If our brains have to work harder to read something than more of our attention is necessitated. The more attention is paid on the front end, the more retention is gained on the back end. My chief concern, however, is that no one will read it at all.
As it is with most cases there is some truth on both sides of the argument. One may be right, and one may be wrong, but there is still something to be learned. Creationists do not negate micro-evolution; Nature enthusiasts do not deign the impacts of nurture; Britney fans do not boycott Christina music; Yooks can eat breakfast with Zooks. Advertising has to be legible, but it doesn't have to be boring! I am a geek, and I like the math behind things. So, let me try to say it like this.
The Retention is proportional to the Attention. R ∝ A The Attention is proportional to the Difficulty. A ∝ D
But the Difficulty has a limit. If it goes too high the Attention drops to 0.
If D ≤ x than A ∝ D | If D ≥ x than A = 0 ⇒ R = 0
In other words
Your advertisement must be legible... enough. How much is enough? It depends on your audience. The font you chose for 32 year old doctors should be different than the font you chose for 16 year old rockers or for 64 year old Veterans. While we're at it the colors, pictures and verbiage should be different too. How in the world are you supposed to know what kind of font/color/picture/text to use where?
It's simple, really. You ask your graphic designer or marketer.