David Henderson gives an overview of a comprehensive study on Wal-Mart:
Using a difference-in-differences specification, our estimates suggest that a new Walmart store actually increases housing prices by between 2 and 3 percent for houses located within 0.5 miles of the store and by 1 to 2 percent for houses located between 0.5 and 1 mile.
Then there is this:
Phone surveys suggest that 84% of households in the U.S. shop at Walmart in a given year with 42% of households reporting to be regular Walmart shoppers (Pew Research Center, 2005). These surveys also show that lower-income households are more likely to shop at Walmart than upper-income households. In fact, Basker, (2005b), Hausman and Leibtag (2007), and Basker and Noel (2009) have shown that Walmart “Supercenters” that sell groceries offer many identical food items as other grocers at an average price that is substantially lower than their competitors. Hausman and Leibtag (2007) also find that these lower prices translate into a significant increase in consumer surplus.
More can be found here.