In this paper, we study the performance of multiuser selection diversity with adaptive transmission based on absolute signal-to-noise ratio (SNR)-based scheduling and normalized SNR-base scheduling. Our performance criteria include achievable system spectral efficiency as well as access probability, average access time, and average access rate of individual users. Closed-form expressions are obtained for these performance measures when the users are subject to independent but not necessarily identically distributed (non-iid) Rayleigh fading and when these users move at possibly different speeds. These results confirm that the system spectral efficiency for both scheduling schemes increases as the number of users in the system grows. These results also show that while the disparateness in the average channel conditions (i.e. short-term average SNR) among users is beneficial to the system spectral efficiency when the absolute SNR-based scheduling is used. On the other hand, the normalized SNR-based scheduling is not fair anymore due to the absolute minimum SNR restriction for reliable communication. For the non-iid case, the normalized SNR-based scheduling has some advantage over the i.i.d. case because of the disparateness in the average SNR among users when the number of users and average SNR for U.d. users are small. This advantage disappears as the number of users and/or average SNR for i.i.d. users increase.