Disclaimer: This originated as a comment to @aeismail's answer but rapidly got too long and complex to warrant being an answer. It's however intended to complement his answer primarily...
I can't speak for Germany or Netherlands but in Sweden you are officially employed as a PhD, thus you do get a "professional salary" and not a stipend, albeit that salary not being on par with what you could earn out in the industry. I cannot say for sure but I have heard that it's a similar system in Germany.
The so-called "industrial PhD"s come in multiple flavours. The type I have seen most commonly are employed by a company which has a certain interest in a particular type of research. Basically by establishing a collaboration with a research group in a nearby university where there might be more resources (time, instruments and most of expertise) they send this student to do a PhD "in-between" the two organisations, figuratively speaking. It's usually a beneficial position for all parties involved, at least on paper, since;
the company gets the research done, with near-minimal costs.
the research groups gets a grad student that is practically free
and the student gets well-paid (often better than her colleagues in the research group) and usually gets better deals through her arrangement with the company in question.
Another way the industrial research might work is, if the company in question decides to have the necessary resources to conduct research in-house. Then you are practically working as a regular employee, you just get to do R&D and publish your results (usually after being filtered to a level which your company is content with).
Finally, about keeping a full-time job and a full-time PhD position at the same time, if you do pursue such a course, make sure you double check your contract(s). Because we are government employed here (in my university for sure, but I think it applies to the rest of Sweden as well) and have pretty decent salaries, we are required to prioritise our research duties. In other words, we are required to not have any other full-time commitments besides our employment. I can imagine that similar fineprint might exist elsewhere as well