I used Underscore a little bit. I thought it had more fluid handwriting than PhatPad (unless that's been improved lately). It was a decent, versatile program, but I did have stability issues which left a bit of a bad impression.
In my opinion, if you use a handwriting app, one of the most important features is how fast and smooth it renders the ink. That's one reason I can't use many of the apps like NoteTaker. It's distracting if it's not as fast as true handwriting, mimicking as well as possible pen on paper. UPad and Noteshelf are excellent for that, as is Penultimate though they might have other reasons that make them inappropriate as everyday note-taking.