  After a short review of the different physics in the weak and strong coupling
regime, resulting in amplitude fluctuation controlled Cooper pair
superconductivity and phase fluctuation controlled superfluidity of tightly
bound local polaron pairs, we suggest a scenario for the cross-over regime
between those two. It is based on the idea of resonating Bipolarons inside a
Fermi sea of uncorrelated electrons, similar to the Feshbach resonance in
Fermionic gases in optical traps. Due to the competition between amplitude and
phase correlations such systems in the cross-over regime can exhibit a
superconductor to insulator transition upon varying the strength of that
resonance.
