Late to the party, as a cheapskate slow adopter of new toys.
Got an iPod touch last Monday, and so far YNAB is the only app I've paid for. It's looking pretty useful if I'm willing to carry the iPod with me everywhere. (I'm still working on figuring out the best way to do that.)
The good: The interface is nice for data entry. I've entered from the account, from the budget, and (my current favorite) from the main screen. I like the fact that when I put in the payee, it remembers the last account I used for that payee. I like the fact that it highlights unsynced transactions; I can use that to update Quicken at home before I sync with YNAB. The sync just plain works as advertised when I'm at home. I don't need cloud sync, but I'll see if that's useful when/if you roll it out.
The really good: When I enter the payee "Walmart Splits" it remembers that this is a split transaction! I wish the desktop version would remember this, instead of giving me a blank category whenever I enter a payee that had a split as the last transaction.
The not-so-good: a) Entering the split transaction is a bit clumsy, but perhaps I'll get used to it. It's not like I have all that many splits; but they do tend to happen when I'm out and about, rather than when I'm sitting at the computer paying bills. b) The small screen shows the amount remaining in each budget category, which is clearly the most important number. I haven't found a way to see the amount spent in a category this month to date, other than pulling up the transactions and adding them with a calculator. I don't think this belongs on the main budget page, but it would be nice if the category detail included the current month's spending total as well as the individual transactions.
Good job on the app, and my compliments to the beta testers who helped you get it to work this well.