I like the pleasant surprise of the beep and then an achievement popping up. I prefer to stay in the dark about achievements and let them occur "naturally" while I'm playing. It's not until I finish the game that I look up what achievements I might like to pursue.
Most developers are smart about it and only throw up an achievement when you do something unexpectedly cool. For example the other day in Crackdown 2 I got my car airborne and then flipped it by accident and an achievement came up. The announcer even said something like "getting a car airborne is one thing; flipping a car is another thing; but flipping a car while airborne, that's an achievement!"