The old torsion beam worked well enough, and obviously the fully independant rear suspension of the newer cars is even better. However, the length (wheelbase) and width (track) of the car also come into play here. The new Jetta sedan is longer and slightly wider than the A5, so it can easier get away with a less sophisticated rear suspension. But being its curb weight is lighter, it will certainly feel a little more jittery over uneven pavement. I have driven them, and if you pay attention, it drives like most any other car with a solid rear axle. Keep in mind, even the luxo-barges from the domestics generally had solid (live) rear axles, and they ride very nicely. So does the Mustang, save for the Cobra version. Remember, Infiniti was selling a $40k+ 'near luxury' FWD car with a solid axle, nobody complained about that, why complain about a $20k Jetta (that is also 10 years newer).
Most people (Americans) purchasing a new compact-to-midsize sedan do not even know what the different suspensions are, nor would they be likely to even notice one way or the other. The new Jetta sedan represents an excellent value, although less so with the diesel. Personally, I think the 2.0L gas version with a manual transmission, which can be had for around $16k, is an excellent car for the money. It is bigger, roomier, and while it may not feel as upscale as the previous model or the Golf, it certainly does NOT feel 'cheap' by any means. Go drive a current or previous Altima with 80k+ miles on it if you want to know what a really cheap interior feels like, or any ChryCo product made in the last ~5 years (Fiat is changing this dramatically, BTW).
I am glad they offer this version, as it will for the first time in a long time allow more people to put a VW in their garage, and that should make everyone that loves the brand happy.