Out of date information now, but I started with a proper pram, that did have a car seat that could fit in the same frame. But I only generally used it for the first 4 months or so with each child. Although it gave my babies some space and a nice flat area to lie onto - it was very big in comparison to the buggies that followed. And it was very rarely used with the car seat fitted into the pram frame - it just didn't look as comfortable for my babies.
I found my Maclaren buggies (I wore out several through lots of use), were great for getting around, into and around shops, onto buses, into the boot of the car, into friends houses with narrow hallways.