By default the preview is not different from P6 but with the right graphics card and when the hardware shading is turned on (on the preview shader settings) the nodes do display on preview.
That doesn't mean the preview shows everything and shows it correctly. Some materials don't show (textures don't always show). Point lights don't show properly (actually lights give a hint about their presence and are not actually very accurate).
Overall preview has improved and after fixing some issues it will probably be even better but it remains a preview.