Public service announcement: for those who don't subscribe to his channel, Dan Smith (the Nibsmith) has just posted a YouTube 'first look' video for the TWSBI Eco. A concise but comprehensive and, I think, pretty fair summary of the pen's features. Take a look if you're interested (and/or feel free to make your own?!).
His review has made purchasing this pen very tempting. He does a great job.