My fairly generic PhD advice, as somebody who did one, graduated a bit early, and is happy about the process and where it's taken me:
* Choose your advisor with care. This is not very easy as an applicant looking at professors' websites, but if you are admitted, any good school will probably have an in-person or virtual admitted students day where you can talk to current students out of faculty earshot. Take advantage of these times to ask about your potential advisor. A truly bad advisor will probably produce at least one person who will warn you about them. If you can't do this in person, try to get a quick phone/video chat -- something off-record where they can be honest. I was always happy to do these for my advisor, because I liked him and wanted him to get more good students. Conversely, I know people who were warned off specific advisors during these events, for good reasons. A bit of subjectivity: a good advisor at a decent school is usually better than a bad advisor at a good school.
* The financial niceness of doing a PhD in field X seems to correlate pretty well with the current job market value of a masters in field X, at least partially for reasons of leverage -- if you can leave and transition into a cushier job, advisors have to provide a bit more value. Computer science scores highly on this metric.
* There is a ton of negativity about PhDs in places like HN. This isn't unjustified: doing a PhD with a bad advisor can be a very bad experience. At the same time, I think "person who had a bad PhD experience" is also "person who writes comments on the internet" with higher probability than "person who had a good PhD experience".
* Choose your advisor with care. This is not very easy as an applicant looking at professors' websites, but if you are admitted, any good school will probably have an in-person or virtual admitted students day where you can talk to current students out of faculty earshot. Take advantage of these times to ask about your potential advisor. A truly bad advisor will probably produce at least one person who will warn you about them. If you can't do this in person, try to get a quick phone/video chat -- something off-record where they can be honest. I was always happy to do these for my advisor, because I liked him and wanted him to get more good students. Conversely, I know people who were warned off specific advisors during these events, for good reasons. A bit of subjectivity: a good advisor at a decent school is usually better than a bad advisor at a good school.
* The financial niceness of doing a PhD in field X seems to correlate pretty well with the current job market value of a masters in field X, at least partially for reasons of leverage -- if you can leave and transition into a cushier job, advisors have to provide a bit more value. Computer science scores highly on this metric.
* There is a ton of negativity about PhDs in places like HN. This isn't unjustified: doing a PhD with a bad advisor can be a very bad experience. At the same time, I think "person who had a bad PhD experience" is also "person who writes comments on the internet" with higher probability than "person who had a good PhD experience".