When achievements were new in TF2, I could reasonably expect to collect most of them, and it seemed fun to do so. Some were so easy, and some you had to put some effort into. But over the years they've added so many stupid achievements, like ones that you could only get on one particular Halloween, that I gave up on all but the most reasonable ones. There was an achievement for the Pyro, a million points of fire damage, that seems reasonable until you see how slow the progress is, and you realize you are grinding. I'm going for a similar achievement in L4D now, the Zombie Genocidest - Kill 53,595 Infected. While I've been trying out campaigns, I've got it as high as 38k, and that took many hours. I feel like I'm already done with L4D, but I want to get this one, and so it makes me feel like I'm not done with L4D. I don't know if you can call it marketing, since I already bought the game. And I don't know if you can call achievements value-add, because I can't measure if I would have been any more or less satisfied without them. But I can tell it does feed the OCD and hoarding sides of me. At least its not making me fill my house up with junk and counting the same things over and over. But I could be playing something else.
At best, achievements help you gauge your level of expertise in a game, at worst they are a treadmill you wonder why you are on.
