Depends on the kid. Some are not very happy in it from the time they get interested in the world, others will happily use it until they are really mobile. On average I'd say you'd get 2 months on the low end and 6 months on the high end out of it, depending on the kid and the carrycot. Also some, like Katie Sue, will be really up and interested if you are carrying/wearing/strollering them, but in the carrycot in the stroller, they will calm down even if they are physically able to pull the head up and wiggle around, because they have associated it with a calm place, from birth on. She loves it in there, I'm sad she's almost too big. She gets easily overwhelmed by sensory input and it's awesome to have a place to put her that's safe from the world.
I'd say if you do a lot of walking, live in an urban or suburban area (where you tend to be walking in and out of restaurants, stores, etc a lot without going back to the car when you're out with baby), or could use a moses basket/bassinet anyway, and it's in the budget, it's a purchase that is definitely worth it. Several companies make carrycot frames that work with carrycots from many brands to use them as a bassinet in the house (if the mattress is firm enough and they are well enough ventilated.)
I'd say if you do a lot of walking, live in an urban or suburban area (where you tend to be walking in and out of restaurants, stores, etc a lot without going back to the car when you're out with baby), or could use a moses basket/bassinet anyway, and it's in the budget, it's a purchase that is definitely worth it. Several companies make carrycot frames that work with carrycots from many brands to use them as a bassinet in the house (if the mattress is firm enough and they are well enough ventilated.)