"I don’t gain anything whit implementing IPv6, it only increases costs"

This argument is related to the (lack of a) IPv6 business case. It is assumed that the introduction of IPv6 will require extra investments. This will in most cases be true: at least someone has to determine the impact of the introduction of IPv6. But the costs can often be minimized by doing IPv6 investments concurrently with the introduction of new network devices and service platforms. As far as revenues are concerned: not be able to deliver IPv6 on time, may lead to missed opportunities, missing potential revenues.

"Is there a roadmap/guideline for that?"

The difficulty of the migration depends on the complexity of ICT infrastructure that needs to be migrated. Basically, a company does not need to know how to deal with the migration, but it should acknowledge the need for a roadmap and know where to look for it. The first step in IPv6 migration is not the migration itself but the decision to start making a roadmap for the migration. A company may choose to either educate their own people or hire external architects and engineers. It is expected that over time migration will become easier.

"When I implement IPv6 in a customer’s network, the network might fail more often than my regular IPv4 implementations. Why should I get myself into these troubles?"

While it is true that there is still far less experience with running IPv6 networks, and IPv6 functionality of network devices might be less mature than IPv4, the assumption that IPv6 networks are less reliable than IPv4 networks is not proven. Timing is very important here, as is knowledge of which products and migration strategies are reliable and which are not.

"I hear people say that Network Address Translation (NAT, RFC 2663) will do the trick. In that case I can keep my current addresses and network infrastructure."

Is NAT cascading, or Carrier Grade NAT (CGN), an alternative for IPv6? NAT does prolong the lifetime of current IPv4 networks, but has issues with accessibility (the end-to-end principle) and scalability (the number of concurrent sessions is limited).  CGN will provide a short-term solution for ISPs who are not IPv6-ready on time. However their total amount of investments will increase, since they will have to move towards IPv6 anyway later on.