This paper considers fundamental limits on adaptive multiple-input multiple-output (MIMO) transmission with imperfect channel state information at the transmitter (CSIT). First, the achievable ergodic rates are derived for both single-user and multi-user MIMO systems with different CSIT assumptions, which shows that single-user MIMO is robust to imperfect CSIT while multiuser MIMO loses spatial multiplexing gain in proportion to delay and inverse with channel quantization codebook size. A single-user/multiuser mode switching rule is then proposed based on the average SNR, the amount of delay, and the quantization codebook size. The operating regions for single-user and multiuser modes are determined, which can be used to select the preferred mode. It is shown that the multiuser mode should not be activated when the delay is large or the codebook size is small.