I have been using this tool to share designs with stakeholders and developers for a long time now (since October 2015). Everybody loves Zeplin and it was so cool to follow the journey of the company and the evolution of the product.
Everything got better since we started using it, but I would like to mention two things that could still use some improvements:
1. Slicing and exporting assets from Sketch produces a different result when compared to the Zeplin svg export (for example, icons even have different sizes) - most of the times we need to drop the files into illustrator and tweak them there to ensure good results.
2. Organizing projects with tons of exported screens is a huge hassle (especially if the project was not organized properly from the beginning). You can use tags and sections to group screens but if you're designing complex products, you will probably find that not to be enough.
Zeplin has transformed our dev team. No more having a zillion versions of a PDF or designers spending a ton of time annotating designs. No more searching a million folders in a million places for assets, no more searching through emails to find answers to questions, and no more debates between developers about how to interpret the design. I love love love having everything in one place in such an easy to use format, I would never go back to the old way of doing things!