DOWNSIDE:

One key disadvantage of deferred rendering is the inability to handle transparency within the algorithm, although this problem is a generic one in Z-buffered scenes and it tends to be handled by delaying and sorting the rendering of transparent portions of the scene.

Another rather important disadvantage is, that due to separating the lighting stage from the geometric stage, hardware anti alias does not produce correct results any more: although the first pass used when rendering the basic properties (diffuse, normal, etc) can use anti alias, it's not until full lighting has been applied that anti alias is needed. One of the usual techniques to overcome this limitation is using edge detection on the final image and then applying blur over the edges.

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BUT you can get around the above - in GPU Gems 3 there is a better one with "edge detection"