For many years I used standard blank or lined-page books as my journals. Now I tend to not even read those old journals because, to be honest, I often went on and on for too many pages about stuff that I could have written on one page. Also, I frequently had a problem using those books because if I didn't write in my journal for a few days or weeks for whatever reason, it created a roadblock. I couldn't write about subsequent days until I had taken the time to fill in all the past days, or else I would have to skip those days entirely. (Or, the horror, write out of chronology!)
For the past 2 years I've been doing something different than in previous years that I am liking much better: I'm using a large Moleskine day per page diary (which I put into my awesome leather Oberon Design cover that my husband got me) as a journal, and this has worked well for me for several reasons:
1) Having only one page per day forces me to be succinct and not blather on. I tend to write more about what I and my family did rather than pontificate endlessly.
2) It also encourages me to write SOMETHING each day, or at least most days. I like to have a record of what I did and I hate having blank days.
3) Probably the most useful aspect of having a dated day per page is that I can go back later and fill in days I didn't have time to write about at the time. (What do I use as a reference when I am back-filling my journal? My planner. Which is a completely separate book from my journal.)
There are certain features of the Moleskine daily diary that make it very useful to me as a journal. Incidentally, some of these features also make it less useful to me as a planner. One of these is the lack of monthly calendars on the daily page. When I can't see where today is in relation to the rest of the month, I can't plan ahead. But for a journal it doesn't matter. I really like the weather and temperature icons at the bottom of each daily page, because I do like to record what the weather was that day and look back at it later. This feature is especially fun now to see what the weather and temperatures were on corresponding days in Moscow (2008) vs. Albania (2009).
But ultimately what makes the daily Moleskine my choice of planner to use as a journal is that the pages look like the pages of a notebook, they just happen to have the date at the top. The subtle lines are the right width, and the space at the top of the page for important events of the day looks just like the top of a regular piece of notebook paper. I guess this is the appeal of a Moleskine: a blank slate for you to fill how you see fit.
For planning, I need lots of structure. For a journal, I still need that daily prompting but other than that I like the blank slate.