PhD at RUG (Work in Progress...)
PhD is a long process (4-5 years of your life!). It can be a very rewarding experience but also frustrating time to time for both students and the advisors. Instead of rewriting what has been very well laid out by many different people, I will refer to the one/s I mostly agree. The guide by Mike Rosulek is an excellent source as the starter and is very detailed. Please take a look at this guide and think about what kind of relation you envision to have with your supervisor (me in this case :)) among these (copy/paste from Rosulek's Web site):
... the mentor is directive in setting expectations, goals, and objectives for the student. The mentor may assign specific tasks such as reading previous research articles and learning to operate laboratory equipment ... the student is unable to advance the research project without direction from the mentor. Stage 1 may be short in some mentoring relationships.
... the mentor still holds the primary responsibility for progress in the research project, but the student may start to make progress independently ... the student should be more knowledgeable about the research project than in stage 1 ... the student should move beyond reproducing results to connecting results and knowledge to a bigger research picture ... the student starts focusing on the "why" of the research project in addition to the "what."
... the student contributes new ideas to the research project and gains an increasing sense of independence ... the mentor and student share responsibility for advancing the research project; the student may take ownership of some aspects of the project ... the student is motivated to advance the project with the mentor's guidance.
... the student has the primary responsibility and the mentor serves as a consultant ... The student is now a colleague to the mentor ... the student is able to make progress on the research project independently ... the student may initiate new directions for the research ... the role of the mentor is to serve as a consultant to the student by providing advice when asked.

This is not to say one relation is better over the other but it is crucial to reflect on this before even starting a PhD.

What does a PhD student do? Reading, reading, reading and writing, writing but also many other things. Read the section "Unsolicited Advice" on Rosulek's Web site.

Why Groningen and RUG? Groningen is in the north of the Netherlands (two hours by train from Amsterdam departing every half an hour) and it is a student city with more than 50K students in most of the year. Compared to Amsterdam, Rotterdam or even Utrecht, it is much more rural and peaceful. The locals are very open-minded and often very colorful :) Yes you are a bit far to the cities where the large companies are located but this makes it an ideal location for a PhD, thanks to reduced daily stress... RUG is a fully fledged university not a technical one. This means besides FSE, in which Computer Science is situated, we have different faculties including medicine, law and social sciences among others, which enable to meet people from other disciplines. RUG is consistently ranked in top 100 among the world.

Salary: There are different PhD schemes in the Netherlands. If you are an employee of the university then you have teaching obligations (often ~10-15% of your time) and your salary would range from 2.3K to 3.2K € as you progress. If you are a scholarship student then you do not need to teach but your salary will be 300-400€ lower. For more details see this.