I'll always use iems, but mainly for the isolation and because they stay in the ears (well, so far only the Titan1's stay in my ear really well).
But with earbuds (at least the Zen and Asura), I really enjoy the feeling of NOT having something jammed in my ears (and/or getting uncomfortable after a while) - I find them comfortable for hours. And I like how easy it is to put them in and take them out. I also get the sense that they are healthier for the ear canals. And as far as sound goes, I really love the huge immersive soundstage. Yup, I'm a bud convert!