Though an important part of privacy, I think this data portability is really useless, for both consumers and businesses.

1. There is no format defined in which the data must be given

2. There is no feasible way of defining such format anyway

3. Thanks to 1 and 2, there is no feasible way of importing this data into another service.

AFAIK, a consumer has the right of requesting the export in any chosen format, but it says nowhere in the GDPR that the data controller must supply this for free [0]. As a business you are allowed to charge a fee to cover the cost of exporting the data, as long as this fee is considered 'reasonable'.

[0] Note: IANAL, a befriended paralegal told me this.