I tried this for a while. I had a few issues. One off-brand DVR seemed to consume batteries quickly and was always dead. One very cheap toy of a DVR had such poor sound that I couldn’t understand the message. One name brand unit worked well and was convenient (I can’t remember the model or brand, but it was probably Sony or Panasonic). These things seem bulkier than necessary, almost the size of a cell phone. A budget of about $35-$50 should get a good one and they are carried at Target and office supply stores.
Once I discovered Jott, I just went with that to minimize the number of gadgets I have to maintain. If Jott is not an option for whatever reason, then I think it makes sense to keep a DVR or something similar (such as a PDA or phone with recording feature) in your car. It really isn’t safe to write while driving, but the car is a good place to think.
I used to scribble the notes right there in the car at the end of the drive so that I could get it into written form and into my system without having to carry the DVR back and forth to the car and most likely leave it somewhere. Plus, I didn’t want to play my notes at work with my coworkers overhearing my reminders to “buy hemorrhoid cream.”