The only benefit of being in Koda Gumi is that you can order priority concert tickets and FC editions of releases. Playroom gets all of the goodies, because it's a closed mobile source, so you can't copy and paste information like you can on a PC. it's much more secure and that's why they get all of the song previews, CD covers, and just random pics of Kuu on there. I was a member of Koda Gumi for a year and didn't bother renewing my membership, there was just no bonus in it for me.
