The uplink performance of massive multiple-input-multiple-output (MIMO) systems where the base stations (BS) employ low-resolution analog-to-digital converters (ADCs) is analyzed. A high performance MMSE receiver that takes both additive white Gaussian noise (AWGN) and quantization noise into consideration is designed, which works well for both cases of uniform resolution ADCs and non-uniform resolution ADCs. With the proposed MMSE receiver, we then employ the random matrix theory to derive the asymptotic equivalent of the uplink spectral efficiency (SE) of the system. Numerical results show the tightness of the asymptotic equivalent of the uplink SE, and massive MIMO with low-resolution ADCs can still achieve the satisfying uplink SE by increasing the number of antennas at BS.