  The B^0_s -> J/psi K_S decay has recently been observed by the CDF
collaboration and will be of interest for the LHCb experiment. It is the U-spin
partner of the golden B^0_d -> J/psi K_S channel and offers a determination of
the angle gamma of the unitarity triangle. Moreover, it allows us to control
the hadronic penguin effects in the extraction of the B^0_d-\bar B^0_d mixing
phase phi_d through a measurement of CP violation in B^0_d^0 -> J/psi K_S. We
discuss the picture emerging for these measurements from an LHCb feasibility
study. While LHCb will be able to extract gamma from the CP violation in B^0_d
-> J/psi K_S, the main application of this channel will be the determination of
hadronic penguin parameters. Such an analysis is actually needed in order to
fully exploit LHCb's impressive experimental precision for the determination of
phi_d from B^0_d -> J/psi K_S. We give also the target regions for the
effective lifetime of B^0_s -> J/psi K_S and its CP-violating observables for
improved measurements by CDF and the 2011 data taking at LHCb.
