Obviously McDonald's, along with Starbucks and others who are following the trend, is benefiting from its prime locations being convenient for businesspeople who now use the places to check their emails over a drink if not a burger. There are a couple of longer-term issues to consider, though.
The WiFi, according to every legal definition, is free. This effectively means McDonald's is paying for it on the customer's behalf. Which means it's costing someone something - so could the money perhaps have gone into keeping prices down or maintaining staff jobs? The answer is quite possibly not - if enough extra people come in to boost profits then fair enough, it's covering its own costs (although as McDonald's is a franchise operation it's not clear whether there's a central repository of WiFi costs and benefits to draw on). But how do the people who don't want to use it but must now realise they're effectively subsidising the smartphone community feel?