I've got a 2006 325, and i've been sticking to the recommended 91 octane.
I agree, the idrive isnt as bad as its made out to be. The salesguy told me that BMW tempered down the original 7-series idrive in a big way and made it a lot easier. I dont use the voice activated commands, and actually rely on idrive for navigation and entertainment and though it took me a couple of days to get used to, i can get around with it pretty easily now.
That said, the GPS interface is not as easy to use as in the japanese cars (and doesnt have as many features). Its has worked for every address i've entered in the bay area the last 2 months except one. But It isnt very good with searching for destinations by name (restaurants etc). Worse, since i dont have the bluetooth package, it doesnt give me the destinations phone number even though it has it in its database. And the menus are structured in a slightly confusing way.
If you get the nav option, the sound system plays MP3 CDs and those are managed through idrive as well and is pretty easy to navigate. My only complaint is that the system doesnt manage song/directory names longer than 8 characters. I like the way you can navigate through radio stations using the idrive too.
Hope this helps,
GL