In this paper, we investigate the average and outage performance of spatial multiplexing multiple-input multiple-output (MIMO) systems with channel state information at both sides of the link. Such systems result, for example, from exploiting the channel eigenmodes in multiantenna systems. Due to the complexity of obtaining the exact expression for the average bit error rate (BER) and the outage probability, we derive approximations in the high signal-to-noise ratio (SNR) regime assuming an uncorrelated Rayleigh flat-fading channel. More exactly, capitalizing on previous work by Wang and Giannakis, the average BER and outage probability versus SNR curves of spatial multiplexing MIMO systems are characterized in terms of two key parameters: the array gain and the diversity gain. Finally, these results are applied to analyze the performance of a variety of linear MIMO transceiver designs available in the literature.