With digital signals, either the digital data gets there intact (without degradation or signal loss), or it doesn't, so if it gets there with a $2 cable, the quality would be no different than with a $200 cable.
HDMI also lets you send the audio and video down the same physical cable, so that's another advantage. The big disadvantage is that HDMI is going to be used to lock down digital devices, so the consumers will eventually lose out in the end.
More info on HDMI is available
here.