We've been using Threadloom Newsletter since the beginning and couldn't be happier with the results. For the price it's a steal. And the company behind it has genuine stated ethics and values. I've been managing our community since 1998, and am so pleased with Threadloom.
Wow, has this been an INCREDIBLY easy solution and win for our site (and our community... and our revenue)!!! Our previous solutions (addons, scripts, Amazon SES, etc. etc. etc.) were a mess and a total pain to manage. That was just on the tech-side. The newsletter curation process was another nightmare all it's own.
Now we can pretty much "set it and forget it" as much or little as we want. We've also had great success selling sponsored newsletter insertions, and we use the custom HTML insertion in the TL interface.
SUPER easy to use and very affordable! Support has also been excellent.