Can't compare against the other models as the v6 is the only 156 I've had, but liked it enough to have bought a second one. Mine are the Q system autos so fuel consumption can be heavy, 22 to 27 mpg, but I do have a heavy right foot which doesn't help. I find it smooth, quick, well behaved in any conditions, just a nice car all round. Expect the manual ones do better on fuel tho.
Servicing and cam belts seem fine to me, just get the water pump and all idlers changed with the belt. Adds to the cost of doing the job but I like the peace of mind. No point to save a few pennies then something fails that throws you a huge bill later.