If you have ever traveled on a flat tire or a tire with very low air pressure, you must have experienced bumps and sort of friction in your body itself. This is in the case where the suspension is still present. Imagine how it will feel if the suspension stops functioning properly. You will never want to drive that car again unless you change the suspensions.
The smoothness is dependent on a number of factors: the main part being the car suspension, the air pressure of the tyres, and the cushions of your car seat. If you are going for a new suspension, you will buy the suspension in pairs. You have to change the suspensions on both sides of the car, else you will get a bumpy drive.
Nowadays the spring-like suspensions are in demand as against the clip ones used in older vehicles. Changing suspensions is not very tough. You need not call a mechanic to replace the suspensions though it is recommended to use the service of an authorized service personnel for the purpose.