We are in the middle of our 34-day YNAB trial. I really like the ease of assigning money to categories and shifting it around as the month goes on, but I'm finding it a little cumbersome to double-enter everything involving a checking account--once in my checkbook register, once in YNAB.
YNAB is my checkbook register. It's great. I can see double-entering data until you are sure you will stick with YNAB. But once you make the leap there should be no reason to keep both systems. YNAB will have all the data (and more) your checking register does.
I've had the same budget file since 2012, so as long as I have my phone with me I can look back that far and tell you exactly how much I spent in any category. I've used this many times when someone asks me how much something cost, "Wait, let me look that up".
How much did I spend at the dentist last year? That's easy to find out.
What day did I open that credit card account that I'm churning? Let's see when the first date on that account is.
What store did I buy item X from? Just review the entries from that category.
How long ago was the dog's last vet appointment? Look back through the 'pets' category.
For big ticket items or orders with confirmation numbers I've started putting that data in the memo field so I always have handy access to it.