  In contrast to the conventional reconfigurable intelligent surfaces (RIS),
intelligent omni-surfaces (IOS) are capable of full-space coverage of smart
radio environments by simultaneously transmitting and reflecting the incident
signals. In this paper, we investigate the ergodic spectral efficiency of
IOS-aided systems for transmission over random channel links, while considering
both realistic imperfect channel state information (CSI) and transceiver
hardware impairments (HWIs). Firstly, we formulate the linear minimum mean
square error estimator of the equivalent channel spanning from the user
equipments (UEs) to the access point (AP), where the transceiver HWIs are also
considered. Then, we apply a two-timescale protocol for designing the
beamformer of the IOS-aided system. Specifically, for the active AP beamformer,
the minimum mean square error combining method is employed, which relies on the
estimated equivalent channels, on the statistical information of the channel
estimation error, on the inter-user interference as well as on the HWIs at the
AP and UEs. By contrast, the passive IOS beamformer is designed based on the
statistical CSI for maximizing the upper bound of the ergodic spectral
efficiency. The theoretical analysis and simulation results show that the
transceiver HWIs have a significant effect on the ergodic spectral efficiency,
especially in the high transmit power region. Furthermore, we show that the
HWIs at the AP can be effectively compensated by deploying more AP antennas.
