I enrolled in the "free trial" yesterday, and decided to immediately forego the trial and start paying for the service - I wanted my immediate spend of $45 for a year's subscription to be a vote of confidence for their awesome work so far (I spend more than $45 per month on silly things like restaurants). I believe worrying about the $3.75/month fee would put me squarely across the line from "frugal" to "cheap".
I have used YNAB4 for 3 years (~$1.50/month) and gotten so much more value out of it. I am sure it's helped me gain insight and save many more dollars than I've spent on it, and will continue to do so in the future.
YNAB's mission and software is something I believe in (it's helped me sway a number of friends and family toward financial stability), and I am extremely happy to pay them monthly for continually improving software. If something brings a lot of value to my life, why not pay the developers for their vision and hard work? (I also subscribe to a SaaS meal planning application, which is great)
As others have pointed out, they haven't completed all features in nYNAB, but I'd rather start using it now than waiting around forever for every little thing to be done. The new "goals" feature is great. Some of the minor fixes and improvements are very nice. Being able to share my login and share budgets with my SO without the hassle of dropbox is great, and will help us to continue to work as one toward shared goals. I have opted to continue to enter all of my transactions manually (it's still possible - no one is forcing automation here), and my SO is trying out the auto-import feature.
I love the new metric "age of money" - it will be a really fun number to see go up over time (and is a great way to measure financial stability for those getting out of the paycheck-to-paycheck cycle).