I've been using Raindrop for about five years. The solo dev really impressed me with his responsiveness to user feedback, so I started paying for premium ($30/yr) even though I don't really need the extra features.

I've used the mobile app, Firefox extension, desktop app (Linux and Windows), and the web app. All have worked flawlessly and have grown more feature rich without ruining the user experience.

The features I use the most are:

• Endlessly nestable folders that I can give my own icons to make them significantly easier to navigate around
• Public collections so I can save my favorite GIFs and memes on a public page I can refer back to and share
• Tagging, descriptions, and notes so I can save a lot of context for each bookmark and why I saved it
• The built in preview for the mobile app feels buttery smooth, so I seldom open tabs in my mobile browser when I'm visiting my bookmarks
• The main premium features I actually use are the broken link detector to find links that no longer exist and the permanent copies of those pages

I haven't had a chance to use the REST API, but it appears to be documented quite well.

Overall I've had such a pleasant experience that I've never looked at any other option.