This paper studies the impact of femto-cell underlay deployment that share radio frequency resources with urban macro-cells. Due to the random and uncoordinated deployment, femto-cells potentially cause destructive interference to macro-cells and vice versa. On the other hand, femto-cells promise to substantially enhance the spectral efficiency due to an increased reuse of radio resources. The performance of systems with femto base station (FBS) deployment is compared to a system where all users, including indoor users, are served by the macro base station (MBS). In addition, the impact of closed-access and open-access femto-cell operation is examined. It is demonstrated that significant throughput gains can be achieved through such FBS deployment, regardless of whether closed-access or open-access is considered. Results clearly indicate that the benefits of FBS deployment by far outweigh their impact on the macro-cell capacity.