According to the best Dutch car magazine, the 159 is a very good car and to give you some quotes:

It is without a doubt the best mass produced Alfa Romeo ever made.
The 159 is in a class of its own when it comes to handling
None of the competitors give the meaning to the term Sports saloon so briljantly as Alfa does.

When it comes to handling:
The nicest handling car on the Nordschleife (old Nurburgring in Germany) is the 185 bhp 2.2 JTS with not too much weight in the nose and therefore perfectly balanced.
The JTD can be oversteered in corners (on the track) and the nose dives a bit much due to the heavy diesel engines.
The 3.2 has a heavy nose as well, but the 4 wheel drive makes the car handle very nicely.