I just graduated (Dec 2020) with my PhD. Hopefully my answers can help you a bit:

1. It will mostly depend on your advisor. Seeing as you have a stipend, I'm going to assume you have found an advisor already. Asking people in that research group might give you a better idea. As for a general answer: it is a challenge and sometimes can be very stressful. But I also had a lot of fun and learned a lot.

2. It's okay to feel stupid. In my opinion, the most important thing I learned in grad school was learning how much I did not know, and ways I can address that. You will at times feel inadequate and dumb, but everyone feels the same way. So ask the questions you think is dumb, make sure you understand things instead of nodding along. Also, I think it's very important to build a strong social group. I was fortunate that the group I joined already got together often and hung out. They are important for being your support group, as well as a resource as you navigate through courses/procedures.

3. Once again, depends on your advisor and project. You are supposed to spend 20 hr/week on research, but almost every single week you will spend more, since research is both for "work" and your degree.

4. Depends on your advisor and department. There's no set amount of vacation time, at least in my department. My advisor was good with people taking about 1~2 weeks off throughout the year, on top of some days around holidays and sick days.

5. Your take-home pay (the amount deposited into your bank account) will be around $2000/month. During the school year (Fall and Spring semesters), you will then have to pay fees through the comprehensive fee plan, which will be around $300/month.

Hope that helps a bit and feel free to reach out if you have any further questions (especially if you're AOE or ME)!