I inherited an old WordPress site, which I've modernized piece by piece. I was afraid to update the Newsletter plugin because it seemed to predict doom if updating. A year and several host and server related issues later, I decided to upgrade. It was completely painless. I had to reconfigure a few things and set up the required fields for the contact form, but it was nothing.
Things could be a little better organized, but it's an awesome plugin - especially for the asking price.