This is definitely useful in the first few weeks. It's so much easier to put the whole seat on the stroller so as not to disturb the baby, plus it enables you to have the baby facing you so you can keep an eye on them.
However, when it's on you can't really collapse the stroller, which is of course the whole point of having a City Mini. It's also pretty darn expensive. You could technically buy a stroller frame for less money, although you would lose the excellent handling of the City Mini. So it really depends on what you need to use your stroller for. If it's only for quick trips on flat terrain, you could probably just get a stroller frame. But if you're going on neighborhood walks on rough sidewalks or at the park, etc., I'd definitely go with the car seat adapter.
Once our baby turned about 8 weeks old, she had great head control so we simply started putting her in the main seat of the stroller with a head support pillow and the seat reclined. While the seat adapter was definitely useful to us, I'm not sure if it was worth $60 to us for only two months of use.