Multi-antenna transmission, or MIMO, is a major enabling technique for broadband cellular networks. The current implementation, however, is mainly for the point-to-point link, and its potential for Space-Division Multiple Access (SDMA) has not been fully exploited. In this paper, we will analytically evaluate the performance of SDMA in multicell networks based on a spatial random network model, where both the base stations (BSs) and users are modeled as two independent Poisson point processes. The main difficulty is the evaluation of the interference distribution, for which we propose a novel BS grouping approach that leads to a closed-form expression for the network area spectral efficiency. We find that the number of active users (U) served with SDMA is critical, as it affects the spatial multiplexing gain, the aggregated interference, and the diversity gain for each user. The optimal value of U can be selected based on our analytical result, with which SDMA is shown to outperform both the single-user beamforming and full-SDMA for which U is the same as the number of BS antennas. In particular, it is shown that the performance gain of SDMA is higher when the BS density is relatively small compared to the user density, but the optimal value of U is almost the same for different scenarios, which is close to half of the BS antenna number.