We investigate the packet-level scheduling for the downlink of multiple-input multiple-output (MIMO) multi-user systems using beamforming. We consider the traffic arrival process and different packet lengths. We tackle low-complex practical implementation that provides low average packet transmission delay and bit error rates (BER) to users. We propose a work-conserving scheduling scheme that considers different users guarantees (heterogeneous users). We implement and compare different MIMO schedulers at the packet level. Simulations show the low average packet transmission delay and bit error rate of our proposed scheduler. Categories and Subject Descriptors C.2.1 [Wireless Communication]: resource allocation. General Terms Algorithms, Performance, Design. Keywords Scheduling, MIMO systems, spatial multiplexing, beamforming.