I reluctantly converted over to battle.net last week.

(I transferred a character between accounts earlier in the week, then when I tried again, all roads led to the battle.net page).

So I created a battle.net account, and added my wow accounts to it one by one. Very easy, very painless.
At that point I then put the serial number in for my iphone blizzard authenticator app.

Since then it has been running perfectly.

Solo play wise I find it inconvenient when I want to swap to a character that was on another account. I have to log out and log back in and select another account from the drop down. It would be nice if they gave this option on the main character selection screen.

I haven't tried it multiboxing - I think if the drop down for which account you want persists then it will be very easy to login (with you broadcasting the code on all 5 logins). Otherwise it will be a bit of a pain to drop down the account x 5 on each one.

My only gripe - I now have to leave my iphone at home with the kids during the summer holidays so they can play wow.
oh - and the app doesn't work on the itouch. I had thought that it might be able to connect via my network to get the code,
and I had hoped that I could get the same serial on both, but the app doesn't start up.