Just a word of caution about ground planes (including virtual): While they have many advantages, as already outlined, and in high frequency work they are almost essential, you should always consider that a ground plane introduces a comparatively large capacitance from components to ground. So if you are dealing with high impedance circuits, say a low current operational amplifier or comparator circuit, you may need to clear the ground plane in the area of high value resistors or low value capacitors to ensure that your circuit is stable and has the frequency response you require. You may even need to clear the ground plane under the chip.
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