The critical bit for many interesting practical applications is this:

    A significant problem, however, arises from the high 
    viscosity of the liquid and the corresponding reduction 
    in its ability to remove CO2.[3][20] All uses of liquid 
    breathing for diving must involve total liquid 
    ventilation (see above). Total liquid ventilation, 
    however, has difficulty moving enough liquid to carry 
    away CO2, because no matter how great the total pressure 
    is, the amount of partial CO2 gas pressure available to 
    dissolve CO2 into the breathing liquid can never be much 
    more than the pressure at which CO2 exists in the blood 
    (about 40 mm of mercury (Torr)).[20]

This has been a known issue since the 50's and shows no sign of going away. The paragraph is a bit muddled--it's the diffusion coefficient, not the viscosity, that is relevant--but the inability of oxygenated liquids to carry away CO2 has so far been the major impediment to doing any of the really cool things this technology has promised for so long.