The IS only makes sense as a 250 auto. If you want a smooth auto V6 it competes reasonably well with the opposition, particularly if you consider the toys/£ equation. It has plenty of faults but then so does the opposition.
As a diesel it is a mile behind.
Forget all this bollox about brands. It is a four door saloon made by the "upmarket" division of Toyota. If you like it buy it but if you are buying because you like the idea of owning a Lexus; get your best friend to give you a good shake before you go out to buy a car!
It is a four door saloon, there are plenty out there, Try them and judge them on their merits as cars. If you go into a car ownership experience thinking you need to purchase an extended warranty you either can't really afford to run it or should be buying a better/ more reliable car.