I have been driving my IS250 F-Sport exactly two months now. This car has been a thrill! For those who like to speed or head to the track frequently, you would benefit from the extra power of the 350. For everyone else, the 250 is plenty of power. This car has tight and nicely weighted steering. Fuel economy is incredible, I average 24-25 mpg with 60% city driving. The driving dynamics are very comparable to the E90 BMW 3-series (2006-2011).
The current F30 BMW 3-series does not feel as sporty.
As for the exterior styling, it does grow on you. I remember thinking how ugly the car looked when I saw the first press photos, but now I think it is one of the best-looking cars I've seen!
Very small turning radius, which allows you to easily make tight U-turns.
LED DRLs and headlights really stand out.
The F-Sport TFT gauges also stand out and are quite functional.
For non-navigation models, the tray in the center console fits an iPhone 5 perfectly (though not bigger phones like the Galaxy S).
The NuLuxe upholstery is very soft and hard to tell that it is not leather.
Relocate the front cupholders; it is awkward to blindly reach back to grab a drink while driving.
Add ventilated seats to the F-Sport package.
Could use better plastics in the interior (it's not as cheap as those found in a Toyota, but it's definitely not the same grade you would expect from Lexus). Make the center LCD display wider.