The buses are quite practical, and efficient. But traffic is heavy, especially during rush hours, so the schedules aren't as exact as the trains. All transportation in Kitakyushu is priced based on the distance you travel. The minmum might be 160 yen but after about 2 kms it jumps to 180, than another km or 2 to 210 and so on. Getting away from Kitakyushu is great.
Ferries are cheap and arrive the next morning.
Ask someone about getting deals on travel in Japan. You'll still be surprised at the prices, even if you can get discount prices. I don't recomment flying out of Kitkayushu, due to the flights being cancelled at least twice per week. To take a train or airport shuttle bus to Fukuoka for 75 minutes and then take a plane to Osaka is for the birds. Take the Shinkansen.