David Lewis is generally regarded as one of the most formidable philosophers of the last century in terms of sheer intellectual firepower. I'm not aware of anyone who thinks he's outright crap. His papers are incredibly well-written - dense, but very well argued and lucid. On topics of interest to LW: he made significant contributions to causal decision theory, the interpretation of probability, the compatibilist account of free will, physicalism about the mind, and the counterfactual analysis of causation.

However, he has been criticized for too often directing his impressive abilities towards an ill-conceived task - the revival of armchair speculative metaphysics. I think this is a fair criticism. Lewis was very adept with logic and mathematics, but he was, as far as I can tell, insufficiently familiar with the sciences, and this shows in his metaphysics.

That said, the idea for which he is most often criticized -- his modal realism -- is now making somewhat of a comeback in the form of Tegmark's Level IV multiverse hypothesis. It's still a fairly fringe and very controversial idea, of course, but its now being taken seriously in at least some non-philosophical circles. It also appears to have some currency among some of the people working on new decision theories here at LW.