Another thing to consider is insurance. One of the reasons I am looking to get rid of my IS300 because my insurance costs over $200/mo (I'm 24 with 1 ticket from 2 yrs ago, but otherwise perfect record) My insurance company quoted me at $88/mo for the Mazda3. Now... apply the insurance logic to an Audi, S2000, etc. Also, an S2000 can easily be eliminated from this discussion by the fact that is an impractical car because it is a roadster.
There is only 2 model of the IS300, sedan or sportcross.... most people get the sedan and it comes in either manual or auto. The IS only has 215hp/218tq and stock will only pull about a 7.3 1/4 mile. In fact, many people who purchased the IS also considered the Mazda6. Yes, one is a Lexus, one is a Mazda.... but the big picture is that you'll still spend A LOT less on the Mazda but get similar performance and options/features.