I think it's completely unfair, single player go for it, but it'd be too much of an advantage for online.

which parts is being considered unfair exactly? just the autofire or the entire controller itself ? if its controller, would the sniper xfps be considered the same? same as any pc gamer who owns a halfway decent keyboard and mouse ?

I think that if I came up against anyone who's player was very obviously aiming that accurately and I could tell, I would set them to avoid.

I ended up picking one up, tested it a little on halo and actually found my game to be worse (if that is at all possible ) whether this is due to mouse settings, or just the fact that I haven't played a shooter with a mouse since postal 2, I tend to think the latter lol.
I'm thinking it would be worth maybe playing through the halo3 campaign again to get myself used to it.
My one big gripe about it is that the config software which must be run on pc, although is actually very simple to use, doesn't allow you to swap x and y axis which would of made the mouse perfect for almost all of the rts on the xbox.