We own a D5 and I can tell you the machine is an absolute jewel. It is a very reliable machine, if this is what you are looking for. The machine is very precise, and unless you have a big crash due to Cam issues, the machine will NOT have to be calibrated. The Tribos tool clamping system is a big plus, because you get no vibration and hence the tool life increases, not to mention the benefits for the spindle. The machine is a miniature German factory, and I know that anyone who has seen the insides of a D5 will agree with me. Once you have a good Cam (we use Sum3D),good tools (we use Datron tools) and the right materials, the machine will not create any problems. The support is very good, and they are happy to give you instructions on how to fix certain issues over the phone or they can connect directly to the machine and check.
The service technicians are very knowledgeable and thorough in checking everything, so after their work is done you can be sure the machine will work properly.
Maintenance is very important as the sensors inside the machine (a looooot of sensors) have a certain life depending on the type of sensors we are talking about. So after a number of hours they will need to be replaced. Bottom line is that if you follow the manufacturer's instructions and take care of the machine you won't have any issues.