I love Sciwheel. I use it exclusively for my referencing (doing a PhD), and live that it stores pdfs and is so easy and intuitive to use. The Word extension is perfect.

Used to be great - but has progressively lost functionality over time. Sharing remains a decent experience, but otherwise has had almost no improvements since release. iPad/mobile experience has also not improved.

I have been using Sciwheel (previously F1000Work) for 5-7 years, and it's the best reference manager I have tried. I started using it because it was the only ref manager for google docs. I don't know if it's still the case. It's easy to use, and the libraries are easy to manage. It's possible to share projects with students and collaborators. The tool has an option to highlight from an article, and it shows the progress in a log form, which it's handy to follow students' progress and to write article's introduction and discussion. Of course, it has all the elements of a good reference manager and provides citation formats from a comprehensive list of journals. Also, the customer service is highly efficient and very open to implementing recommendations from users.