Are you expecting, Nohayo?
I have a car seat stroller combo, but I never used them together. It's great if you are going to go out a lot and need a stroller when your baby is young. You just pop the car seat out of the base and into the stroller. I just never used the stroller until recently, and Gabriel is old enough to sit in the stoller now.
Now those carrier car seats (these come as the stroller combo or by themselves) are a must-have in my opinion. It is so much easier to carry a baby in them, especially when they're young and you have to go in a store. You can't sit a small baby in the cart. Plus, if you're baby is sleeping, you don't have to wake him to take him out.
I also have an umbrella stroller that I use from time to time. It is much cheaper and smaller than the other strollers. The only down side is that young babies cannot sit in them b/c they cannot sit up yet.