Yes, I keep a detailed diary, as an Excel spreadsheet. I record all medication changes, PSA test results, all appointments - who with, summary of what discussed, and actions. After almost 2 years and an untypically complicated diagnosis phase, it runs to 3 pages when printed out. Actually, I keep a second one too which is much more detailed and includes day to day symptom changes, details of each RT session, etc. This one now runs to 11 pages.
I also keep all my blood test results in a spreadsheet, and medications in another, and take spare printed copies with me to appointments, several copies in the case of inpatient stays. As each different clinician asked me about my meds, or eGFI, or whatever, I could just give them a printout, and I got so many responses "Wow, I wish everyone did this".
I've also found them useful in my work supporting other patients. I was doing a presentation to other patients "Surviving Hormone Therapy" on Friday, and I got a very typical type of question, "How long before you lost your body hair?" A quick check in my spreadsheet diary, and I could answer that it started at 17 weeks, not the sort of thing I'd have any chance of remembering a year or more later without the diary.
My consultants, and other consultants I work with who aren't mine, have occasionally asked me questions, knowing I keep such detailed information.
I think for me it's also part of being in control of my disease, something that's psychologically important.