Manchester united with Ryanair again, but at what cost?

Ryanair announced plans to open its 45th base at Manchester Airport in Oct-2011 in a claimed GBP175 million investment, basing two aircraft at the airport and launching 17 routes, increasing by summer 2012 to four aircraft and 26 routes. Ryanair has been there before, then pretty quickly left following a dispute over charges (apart from the perennial Dublin route). This time it is committing to a base operation but what is the potential downside to Manchester?