Just joined to say thank you for making such an awesome backup solution!
I have a personal Nextcloud server, self-hosted on home computer, and I’ve been trying to switch away from other hosted services (gdrive, dropbox, etc) in an effort to regain control over my data, but the missing piece to complete the switch was always the fear of losing all my data (hard drives fail, stuff happens).
I’ve been looking around for efficient, easy to use, open-source (so I can trust it) backup solution and duplicati is the only one that ticked all the boxes. It took some ~17hrs for the initial backup to S3 (~130GB over not-so-fast home connection), but now it routinely takes 5 to 30 for incremental backups. Projected monthly cost: $2-$3 / month. Awesome!
Thank you and please don’t stop developing and improving it! Here, have some of the money I saved over any commercial, hosted solution (on your way via Paypal) 
-Daniel