Agree with previous poster, Balaton isn't the best as investment, it's ok if you want a holiday home for your own summer use only. The place is dead in winter, as some posters said its just a summer season thing. The summer does extend a bit with weekend traffic, but the place is overrated quite honestly.
During the big rush by foreigners to Budapest a few years ago, when people bought the bad along with the good, a few agents tried to revive the Balaton market for sale to foreigners, but were generally unsuccessful. If it couldn't be sold then, when selectivity was not an issue, it looks like a bad bet now that the amrket has stabilised -- no chance of getting customers if you want to exit.
As for Ryaniair flying into the old military airport in Balaton, this will have no impact on the local property market; 99% of passengers will jump on the bus for Budapest. Just as in Charleroi, where eveyone goes straight to Brussels, the local market impact will be minimal.