Hi mate, I picked my GS300 Sport up on friday so I'm a new owner, I have gone from a e46 325 Sport to a Lexus, totally different cars though.
I am getting about 26 mpg from mine and had it upto 32 on the m/way, not as bas as I thought it would be to be honest.
It goes quicker than I thought it would as well which is a bonus.
Mine is silver pearl colour unsure of the correct name for it with black leather and sat nav.
They are lovely cars to drive, very smooth and seem really well put together as good as my 3 series if not better and come with a great spec also.
I thought the stereo would be better in it given the number of speakers, sub and a amp they come with, this is a bit of let down.
One of the reasons I went for one is that they aren't a Audi, BM or a Merc that you see every tom, ***** and harry driving. You dont see many GS's on the roads imo.
Good luck with finding one, make sure you pick a nice colour as well.