I have always liked them from i kid also. I caught up with a old school buddy not long ago and he wasnt suprised i got one as he remembers i always used to look at them back in 1999, 2000 etc. I wanted one at 23 but couldn't afford the insurance so at 25 i made the plunge, a little bday present to myself lol. funnily enough my previous car needed new tyres all round which and i thought the same thing, either spend £200 on tyres or get a IS200. wasn't a very hard decision for me lol. I was a little worried about MPG but i only do 6k a year so i thought what the heck i can afford it. I have the auto, i get 300 - 325 miles before the light comes on from mostly town driving which is about 22mpg, i can get 450 - 500 on a long run very easily. Basically these cars are best suited to the motorway, A and B roads, not that i care i do mostly town driving in my IS200 auto hence why i have a auto. Not sure why you would think a BMW 320 and 325 would be any better both 6 cylinder engines. maybe 1 or 2 MPG better but probably no different plus less reliable.
Anyway based on everyones input on MPG figures im sure you can figure out how much more your fuel bills will be. the tank is 75L btw Anyway welcome to the forum and keep us posted :)