I will never get why some people care about achievements, just play the game and enjoy it. If after finishing the game you want to do the achievements to get "more" out of it that's cool in my book though.
It depends.
- The grindy ones (kill x number of things using y weapon) are of pointless
- The random, stars-aligned-just-the-right-way ones are kind of pointless (a lot of TF2 ones)
- The milestone ones are interesting to see globally how far people have gotten in a game
- The hidden ones give you hints that you haven't as thorough as you thought (Steam will tell you the total number of achievements for a game, but won't show you the descriptions of the hidden ones)
- The challenge ones push you to improve or encourage you to try difference approaches
The only games I've 100%-ed are Orcs Must Die!, Costume Quest, Sequence, and Droplitz -- ones with maybe a few dozen at the most, and ones that encourage thoroughness or challenge, without being too grindy.