Reality: The fact that you have no defined work hours means that it is far too easy to spend all of your time: working, pretending to work, feeling bad that you aren't actually working, feeling guilt about how much work you still have to do and knowing that since you have no defined work hours you could be working right now, you slacker.
The reality is that with academic work, there is always more to be done, and the fact that there are few defined work hours and there are no concrete "end points" to work like writing, editing, and revising research papers and class presentations means that you will be working constantly, or feeling guilty about any amount of time you spend not working.
Something else that no one tells you is that for as "flexible" as the job is, there is still an expectation that you will be on campus regularly, for a lot of scheduled activities. Classes that you sit in and teach, seminars, colloquia, regular meetings with advisors, etc. You will, basically, be expected to show your face on campus on a regular basis. If you would prefer to skip the seminars to stay in your office and really get some work done, that's fine - but if you start doing that too often, you will be considered to not be truly dedicated to your work. Even though you are at home working on it.
But, okay. You won't punch a time clock, and you're free to schedule a doctor's appointment during the middle of the day or to skip the Friday seminar to take your daughter to the zoo. That is (mostly) true.
But what they don't tell you is that while the schedule may be flexible, it has an evil downside: work with no defined hours has no defined end time either.
You will see grad student friends on Facebook talking about how they're just leaving the lab at 11pm, and you will feel bad that you are sitting at home watching a movie with a glass of wine.
You will talk to other grad students about the 15 projects they have going, and feel bad because you only have 10. So you will take on another project, or agree to teach another class, or simply feel bad about yourself and start working longer hours to finish up whatever project you're currently working on that's closest to completion.
But even when you "finish" a task, it's very likely that you are not really finished with it. The paper that you sent off for publication may be rejected, which means you need to revise it and send it back out. You will have to revisit your "finished" lecture later to revise and update it. Etc. etc. etc. It will often be years before you can truly "finish" a project.
And on any given day when you are working on a project, it's very likely that you will have nothing concrete to show for it on any given day. You might run statistical models that don't yield good results and are useless. Or you might spend the day reading research written by others for your own literature review. Or you might spend half of your day writing the first draft of the exam you're going to give your students. You come home, and your partner/friend/roommate/dog asks what you did all day.
And you realized that you accomplished nothing concrete. So you feel bad, and you head back to the computer after dinner to "be productive already."
So much for free time and flexibility.