From Ryanair's point of view, it makes a lot more sense to operate to STN than LGW given their size of operations at STN, and the move will give BFS a second 'hub' connection (after LGW with U2/DY) given that Ryanair will soon be rolling out single ticket connections via STN in the next year or so.
From the London business traveller's point of view, this is bad news, given that LHR, LGW and LCY are the preferred airports for these travellers. However, if the rumours about IAG regaining the LGW slots are true, we could see the resumption of BHD-LGW which would benefit the business community a lot more than Ryanair's BFS-LGW route.
From the budget/leisure traveller's point of view, Ryanair's move will initially provide more choice, but may well lead to easyJet increasing fares on BFS-LGW, especially if IAG don't resume their LGW flights. And of course, given that easyJet have only a small base at STN, they may scrap their BFS-STN route instead of fighting Ryanair, thus leading to a reduction in seats available between Belfast and London.