I've had MacJournal less than a week, so Mike's (Mariner1) comments above will be much more valuable. But, like you, I don't drink beer or have a sister, so maybe my views will help. I downloaded a demo copy a few days before nanoBundle2 began, decided to buy it soon, and then saved some money by purchasing it from MacHeist.
I downloaded MacJournal for basic journaling. I had tried keeping a journal with online services and with other software. MacJournal is the first application that feels like it's the right tool for the job. Perfect.
I've read comments elsewhere from people who said Evernote is just as good as MacJournal for journaling. I love Evernote as an "input box," but it lacks the ability to format text well enough to serve as a journaling tool. Despite some obvious similarities by organizing with journals/notebooks and tabs, there are a lot of other differences that make each of these great pieces of software appropriate for different jobs.
As a professor, I briefly considered using MacJournal for organizing my course plans. But I've always used applications with outlining features for this, such as OmniOutliner and Word's outline function. MacJournal wasn't quite right. (I have read of other teachers who love using MacJournal for class plans.)
However, MacJournal has met a need in my job in an unexpected way. I teach music composition, and I've never had an acceptable method of keeping notes on each student's lesson. MacJournal has been ideal. I have a journal for each student and I create a new entry after each lesson. I can drag notation files, pdfs, and audio files directly into each student's journal, so my notes and their work are organized by tabs and MacJournal's calendar. Before each lesson, I review my notes from that student's previous session. I've tried a lot of other software for this, but nothing has fit my needs nearly as well as MacJournal. It has made a difference in my work after only one week, and it will be invaluable after a few semesters when I use it to look back over a student's work. Obviously, this specific way of using it will not be important to many people. But great software always meets the intended need and eventually helps in unpredictable ways. Like me, you'll probably find some unique uses for it.