Beyond diagonal reconfigurable intelligent surface (BD-RIS) refers to a family of RIS architectures characterized by scattering matrices not limited to being diagonal and enables higher wave manipulation flexibility and large performance gains over conventional (diagonal) RIS. However, whether BD-RIS aided network is able to deliver more data volume compared to conventional RIS aided network is still questionable, because far more time may be wasted to estimate the massive channel coefficients associated with the off-diagonal entries of the BD-RIS scattering matrix. Amazingly, for the first time in the literature, this talk will show that the uplink channel estimation overhead in BD-RIS assisted multi-antenna and multi-user systems is actually of the same order as that in the conventional RIS assisted systems. This fantastic result is based on some channel property that has not been revealed before. The message is that we can benefit from the non-diagonal scattering matrix design at a channel estimation cost similar to that in conventional RIS aided network.