Yes, I keep a journal. It has been a very helpful tool for me, not just now, but through many times in my life.
I often find that what I write in my journal helps me process my work with my T. Sometimes, it stays in the journal, sometimes, I end up finding I have specific questions or topics that keep coming up, and I take that as a hint to address them with T. There are some entries in my journal that I have never gone back to re-read (I mark them with a specific character) as they were very raw and emotional, and I'm not in a space to revisit that experience.
When I'm particularly stressed, I actually write letters to my T in my journal. Occasionally, I will end up reading a letter to her, but usually, they are just another way for me to get things out. I sometimes end up writing a series of letters during the space between appointments, and will often take and condense them into one letter the night before an appointment, to help me focus on what I want to bring to the table to discuss. Sometimes I end up reading excerpts of that letter, other times, it just helps me to get focused and prepared for an appointment.
After my appointment, I like to use my journal to capture any bits of wisdom I don't want to forget (I usually do this in the parking lot before I've even left!) and then later, may go back and try to summarize the appointment, or at least capture how I was feeling, and what might have brought that feeling out.
All that said? Journaling is just a tool for me - I need my T (as much as possible these days!) and journaling does not by any means replace that. It helps me to stay a bit more in the zone between appointments.
I am using an online journal that is actually encrypted with 24-bit security, and if I lose my password, I lose the journal, because the company can't even access my account (same strength as military security!) It's great because I can access it from my iphone
Here's a great link about journaling by Ryan Howes on Psychology Today's website:
Journaling In Therapy (I find his blog, In Therapy, very insightful!)