Beppu was pretty sulfurous; I went this past spring on a Kyushu tour (Fukuoka-Hakata, Nagasaki, Saga, Kumamoto, Mt. Aso, etc.) and Beppu was the least favorite part of that tour for me. It does have a lot of hot springs, tho. It depends what you want out of the trip.
In all honesty, I don't know how the weather would be in Kyushu this time of year, but I'm sure there are weather sites you can check before you go. Try Yahoo if you can:
Japan - Asia Weather Forecast on Yahoo! Weather
Good luck!
