I've been a user on and off since 2007, and for 2 years 2 months straight this time around. I have posted a while ago about all this, but I'm still so pleased that I'm posting again.
Although I already had some of the principles, (esp saving for annual bills) it has got me through some pretty big changes. Getting a new high-paying job and not spending the money on ridiculous things. Getting sick of high-paying job, and saving for a freedom-fund so I could quit and re-evaluate my career. (I actually named the category Freedom) Got me through being unemployed for an extra 3 months than I planned due to my mum having a major operation. I never once failed to contribute to the household income all the way through that period, although it was somewhat reduced. Got me saving for training to become a self-employed bookkeeper while I was temping.
And now I am out on my own in self-employment, it helps me plan spending while I have a variable, but luckily increasing, income. It gives me the peace of mind to see that I am putting 25% of my income away for taxes (I'm in the UK and have been used to all my taxes being deducted at source). We are about to move in a house from having been in our little flat for 8 years, and YNAB will help me put together a household budget for the first time; we can keep an eye on increased outgoings whilst building our depleted savings once again to pay for a new kitchen and extension; help us plan for my reduce income when we have kids.
It's such a powerful tool and it's easy to forget how much peace of mind and control it gives you over your money. No matter how much or how little you have. If you are on the fence, buy it. You will not regret it, whatever your current situation.