I've managed to stick with this bullet journaling thing for just about three and a half months now (see my first post here), and so far, I can say that I'm enjoying the process! I've made a few modifications from the original system since I started, and will probably continue to do so as I customize it to best fit my own lifestyle.
The first customization I made was to change the monthly layout to a more traditional calendar view. I wasn't a huge fan of the list since it ended up being a little messier/cramped than I like, and it was difficult to show events that spanned several days. I also like seeing at a glance what day of the week it is, and how the weeks relate to each other. Some bullet journal users claim that the calendar view doesn't give enough space to write down events, but I didn't have a problem with that. And it's kind of fun to draw boxes with a ruler. Here's what my calendar looked like for October (note the use of arrows to show multi-day events):
I also added a habit tracker to my monthly spread to keep track of stuff that I should be doing daily. It's basically just a list with checkboxes for each day. I'll admit that I still haven't been great at checking all those boxes (i.e. I still don't make my bed every day), but it has helped me be a bit more accountable.
Besides that, I've added a few more lists: stuff to buy, tutoring contacts, fun quotes, and birthdays and anniversaries. Although my current dollar-store notebook has served me well for learning how to bullet journal, I'm enjoying the process so much that I've even already invested in a fancy new notebook: a Cottonwood Dot Matrix notebook. It's gorgeous and has a built-in pen loop! (I tried to make my own pen loop out of tape for my current notebook and it looks pretty crappy.) I can hardly wait to try it out at the end of the month!