I like them enough that it's a little depressing to play through a title on another system now and not get some sort of little trinket for accomplishing things.
And I enjoy the ones that are just a little out of the way, too. In Crackdown, for example, during the course of my gameplay I found about 95% of the agility orbs. Will I go for 100% and the achievement? You bet.
I only found about 30% of the hidden orbs, though, during my normal gameplay. Will I find the rest of those just to get the achievement? Nah.
Continuing the Crackdown example, I think the "do this" achievements are pretty fun most of the time, such as "Get a car 200 ft. high" or "Get to the top of the Agency tower and jump off without dying." Nothing you'd do in the course of a normal game, but fun little challenges left by the developers.