Members on m5board arent complaining on uncomfortable seats or stiff suspension. Like PC said, the two main complaints are SMG in automatic mode and fuel consumption.

The seats in M5/M6 is out of this world, very comfortable (which carmags also pointing out), in fact they are just as good as the ones in E55. The difference between M5 and E55 is the suspension. The suspension in M5 E60 is not like M3, M5 E39, RS4 or various Porsches but harder than E55.
The suspension works good for daily-use and transporting 3 or 4 people work as well, much better than the cars I listed above. Put the car on Autobahn and you have great transport-car.
Again, its the SMG that is the mainconcern. Some like it , some dont. Luckily BMW are listen to their customers, both manual tranny and SMG will be offered for the M5.

We also know for a fact that BMW are working on gearbox similar to DSG (which is better for daily-use than SMG), if DSG will replace SMG in the future, that remain to be seen.