question about carbon seatposts.... I think I want one :)

[Duck on Wheels]

Also are carbon seatposts likely to crack? what is their lifespan?

Yep. Carbon seat posts do crack. I had one that lasted a year, riding moderate distances on mostly fairly smooth roads. I did hit one pothole a couple weeks before the seatpost snapped. That may have been a trigger. I've replaced the seatpost now. Not carbon, although the carbon post did make for a smoother ride.