Note that I am biased. I know what I like in a car and although the 156 certainly has its faults, I do like it a lot.
Here is a cleaned up version of my email response:
Clarkson is getting old. The 159 is a high quality car and it looks a lot better than any german car -- inside and out, but I found it rather unexciting to drive. it is too heavy and the steering isn't the way I like it.Now, in abslute terms, the 159 is a good car. The build quality is very good and it is hard to think if any non-italian car similarly priced that is as beautifully styled.
Then again, I like the 156 with sports suspension and 17" wheels. A lot of people don't like it.
The ride is a bit harsh on mine. In part because I have aftermarket polyurethane bushings (powerflex). The 2 litre engine is nice, but not great (about 100Nm short of being exciting) and the steering is razor sharp so you have to be prepared to a) have the suspension adjusted every 2 years, b) the car will respond to the surface it drives on, and c) your inputs are translated directly into action so you need to calm the f*ck down when driving it and be precise. The latter takes a bit getting used to. You can forget about nonchalantly holding one hand on the wheel while driving fast on uneven tarmac in Norway. The steering behaves like a border-collie: eager. (In fact, it is noticably sharper than on the Nissan 350Z)
Also, the 156 is very light. the downside is that you really notice that the engine has only 155hp if you have more people in the car (larger percentage of the total weight being variable). I don't think most people actually notice that the balance of the car changes, but I do.
The V6 version has more grunt, but it comes at the price of a heavier nose. On a twisty road I drive faster in a 2-litre 4cyl than I do in a V6. In part because the front tends to understeer badly and in part because I haven't driven the V6 enough to figure out how to compensate. (Any front wheel drive will tend to understeer when driven "naively", but there are certain tricks to dealing with it. The problem is that the more unbalanced the car is by design, the more violent the countermeasures).
I also noticed that the Recaro seats that came with my 156 are very good for active driving. they are narrow and have pronounced sides so the seats hold you in place. the only thing I am missing on longer trips is adjustable lumbar-support. The 159 I tried has adjustable lumbar support (I couldn't figure out how to work it in the limited time the car was available to me) and possibly adjustable width, but I didn't feel like they held me in place well enough. Comfortable, but not made for sporty driving.
Steering wheel on most new alfas I've driven are great. Thick and chunky and usually relatively small. The number of turns lock-to-lock varies a bit, but the 156 can be driven without changing grip. The 147 has even fewer turns lock-to-lock. Turning circle is terrible though. Not much room inside the wheel-arches for those (relatively speaking) big wheels. Which can be a bit of a challenge when parking.
The brakes seem to be varying a bit from car to car. in general it is my impression that the 156 has very immediate bite and then a more progressive bite as you push the pedal further. First time you drive it you'll probably apply the brakes a bit too hard and end up with your face in the windshield. I think this is a property of the braking servo setup rather than the brakes themselves. The standard brakes are made by Brembo, but I think that the quality of the pads varies a bit. (Mine are starting to go now, so I am planning to mount Black Diamond grooved discs and Ferodo DS 2500 pads all round. according to what I've read, slamming the brakes fully on with this combo is so effective you have to expect bruising from the belts).
As for tyres, I have no idea what the standard stuff is, but I drive on Pirelli PZero Nero. Those are a bit expensive compared to the regular cheap stuff, but very good road tyres. I was driving on rather expensive Yokohama tyres earlier and while they didn't disintegrate the side-walls were a bit on the soft side and when they got worn they got a sharp'ish edge that made them a
bit too nervous on uneven roads).
Overall, the 156 with sports suspension and wide tyres has a very edgy, sharp feel to it whereas the 159 is about 250kg heavier and feels more..."homely" to drive. The 159 body does have higher torsional rigidity, but the less sensitive steering and the softer suspension (I haven't tried a 159 with stiff setup) makes it feel softer and less responsive. again, this is a matter of taste. Some people think my car is quite a handful to drive and is too responsive.
But again, I like hard cars. I don't mind harsh ride, I don't mind that any steering input has a very direct and precise response.
159 is probably the highest quality car Alfa has built in that segment, but it is also a very dull car compared to the 156. compared to other cars in the same segment it is still both good looking and a good drive.
If you don't understand a word of what I said in the email response republished in the above paragraphs, then the 159 is for you. :-)