I'm going to start this off by saying that carpooling is still better than taking two individual vehicles to and from the same area every day. Don't forget that if you can carpool, and you still ride (in a car) alone, you're riding with Hitler/Bin Laden/Microwaving the Baby.
So, I'm all for the reduced fuel needs, wear and tear on roads, decreased parking needs, etc, not to mention the pleasant company in the morning.
On the other hand, since I've stopped riding my motorcycle to work and started carpooling, I've gained 20 pounds.
I work as a computer programmer, and my job consists of sitting and wiggling my fingers a little bit every day, while drinking ungodly amounts of coffee. I know, pretty sweet gig. The catch is I do not move around during the day.
When I rode to work, I had a five minute walk to and from my parking garage, plus several flights of stairs, not to mention the activity of actually riding the darn motorcycle. Now I don't even have that.
My work provides a gym, I just have to sign up for access. So, I guess I'm going to do that. And one of these days I'm going to actually sign up to train at the local boxing gym. In the meantime, I've got to stop making excuses and haul my bicycle out for rides after work.