It is a good car, when compared to other cars.
But when compared to Lexus itself, this particular generation of IS's have less than stellar reviews from both users, and reviewers. Most complaints come from the low quality interior components which are not so nicely assembled across the dashboard. Compared to GM, its a whole lot better, but when compared against Lexus and Toyota, the quality is a couple of notches below.
The ride is also less refined, when compared once again, to Lexus and Toyota.
But what makes up for the less than stellar quality and unrefined ride, is the price-value factor of the car, It is quite a bit cheaper than BMW and Mercedes whilst offering a sporty fast car(although it will still be slower).
But comparing disadvantages against advantages, the advantages outweigh.