We present an efficient high-power planar waveguide laser consisting of a 2% doped Yb:YAG core with sapphire claddings. The planar waveguide is double-side pumped at 940 nm using two custom aberration-corrected diode laser stacks which are imaged directly into the waveguide core. A method to measure the pump uniformity has been developed, showing uniform pumping throughout the waveguide core. Laterally resolved gain measurements show a uniform gain of 1 cm-1 across the full width of the device and the onset of pump saturation. Visible emission from 475 nm to 550 nm is observed, which is interpreted as due to a cooperative luminescence process. Using a plano-concave resonator, 400 W output power is extracted with a slope efficiency of 75% with good transverse beam quality but poor lateral beam quality. Using a hybrid unstable resonator, 300 W output power was achieved, with significantly improved lateral beam quality but reduced transverse beam quality due to interference from cladding modes.