What makes a great radio commercial?

There are a lot of crap radio ads, the ones where they shout at you, the ones that try and use humour (unsuccessfully) and the ones that irritate and bore you in equal measures.

So what makes a great radio ad.  Martin Sims who is the Creative Director of Eardrum and former Creative Director of LBC Radio: 

"For me, a good radio ad has to tick three boxes. One: engage or entertain the listener. Two: link it to the brand. Three: effectively communicate some sort of commercial message. If you tick all three of these boxes, then you're on to a winner. If you only tick two, then you'll generally have a very funny ad that's unrelated to the product or an ad that's very product-focused but as dull as dishwater to listen to.

"Creatives are getting more excited by radio. With reduced ad budgets and the proliferation of TV channels, creative teams are realising they can get noticed by doing two or three radio ads. It's an opportunity to shine."

(Originally from email comments in Campaign 2006)