JR Pass or Kyushu Rail Pass

Will be in Osaka for two days in mid Nov b/4 going down to Fukuoka (by jr or shinkansen?) I have 6 days in Kyushu and wonder if its a good idea to take JR fr Fukuoka to Kagoshima and then do a self drive around Aso, Tachikiho Gorge, Beppu, Kurokawa areas. Will it more cost effective to just self drive and forget abougt Kagoshima? Thanks

Where do you have to be at the end of the 6 days? Back in Fukuoka? Somewhere else?

Will be in Osaka for two days in mid Nov b/4 going down to Fukuoka (by jr or shinkansen?)

The shinkansen are operated by JR! Here are you various options for travel between Osaka and Fukuoka:
http://www.japan-guide.com/e/e4802.html?aFROM=2157_2161

Renting a car from Fukuoka for driving around the rural central area of Kyushu sounds like a very nice idea to me.

After six days i.e. on the 7th day I 'll be back in Fukuoka for flight back home.

In that case Kagoshima cannot be elegantly included in the itinerary but would have to be visited in an expensive side trip. Whether you want to do it or not depends on your budget, time available and interest in Kagoshima. 6 days would be enough to include it alongside Aso, Kurokawa, Takachiho and Beppu.

Time-wise Kagoshima can be taken it but the question is whether it is worth the extra effort plus expense (if not using a rail pass) and that your other planned destinations are mostly in the north.

Kagoshima is indeed worth visiting and is the perfect base for a tour of southern Kyushu. You can spend many days there with side trips to Sakurajima, Ibusuki, Chiran, Kirishima, Ebino Plateau or even venture east to the beautiful Nichinan Coast near Miyazaki.

With 6 days a self-drive loop of the northern part of Kyushu from Fukuoka should reward you with a more leisurely trip. Besides Aso, Beppu, Takachiho Gorge and Kurokawa Onsen, you can take in Yufuin, Kumamoto and Nagasaki. If you still have extra time, consider Shimabara, Unzen, Yanagawa, Dazaifu and Mojiko.