Rainbow Islands Extra on Megadrive is nice and all (I own it due to my love for the Bubble Bobble series), but it does not carry over how mind twisting Extra in arcades was over the original Rainbow Islands. in Extra arcade you could actually alter which special item you wanted in the secret rooms if you got the diamonds on an island in exact order. In RI itself the items in the secret room never change from game to game, the MD version of Extra seems to retain these features. The codes for Extra arcade are also different from the original, and the MD version doesn't use these codes, but I have only ever found 3 working codes for that port, speed shoes and a couple other minor powers.
About RI and RIE's music and how it got altered for all modern RI/RIE releases (except the PC port of Bubble Bobble and Rainbow Islands which didn't get anything altered, here's how it sounds on Taito Memories:
http://www.youtube.com/watch?v=uCK-wAumY-U
Now how complicated Extra is, well the whole item altering in secret rooms thing is explained in detail here, lines 60-158, quite complicated as hell if you ask me:
http://tech.quarterarcade.com/tech/MAME ... spx?g=1386
Edit: The MD port of Rainbow Islands Extra sadly has some music missing, one I can remember off the top of my head is the unique music for the boss Electric Fan of Darius fame.
Edit part deux: I second BryanC's recommendation of SMS Bubble Bobble, you will never look at the NES version the same ever again after playing it. I love the NES version and all as I played it many a years ago and now own it and the second NES title (and the GBA one, and the 2 GB ones, and RIE MD, and Taito Legends and Memories, and the Saturn Bubble Symphony, and the PCE CD Rainbow Islands, and the SMS Bubble Bobble complete....yup I love Bubble Bobble and it's Puzzle Bobble spinoffs up to PB 4 and the arcade Super Puzzle Bobble), but the SMS Bubble Bobble is closer to the arcade version, and has 200 levels compared to the NES's 120 something. Yes levels 101-200 are exact replicas of levels 1-100, but they are all in the dark unless you found the games hidden candle in a bonus room. Love love love love this series to absolute death, just missing Parasol Stars to finish the series, thankfully Taito put Symphony and Bubble Memories on the last 2 Taito Memories compilations, as well as Bubble Bobble and both revisions of Rainbow Islands.
Looks like I went on a tangent for this series, not often do you find people talking about it.