Our small design firm has used Figma a few times on large apps with hundreds of screens, and we prefer it to Sketch.
It's smooth in the browser, although we use the desktop version which runs remarkably well on Electron. Files auto-save instantly and you can do all of the fancy animations and effects that Sketch offers.
The killer feature for us is inviting clients or other designers into the document whilst editing it in real-time like Google Docs. Clients can lock themselves to your viewport so you can talk them through the design (huge win for me, the clients love it), or they can browse around freely and edit the document if you allow them.
Figma launched in 2015 so they are relatively new. I don't know what their marketshare is but they definitely have 'mindhshare' among many designers (if that counts for something).
If you do an online search for "Sketch vs Figma" (or on YouTube) you'll find dozens of comparisons. Figma is browser-based which is a turn-off for many people (including me). However, sceptical reviewers have been won over by its features and the UI. So in the design tools space, they are definitely one to keep an eye on.
I've switched from Sketch to Figma, and I'm quite happy about it. I'm also using Affinity Designer, which is a fantastic complement to both.
But I'm odd—before Sketch, I used Antetype. I really miss the unique layout engine of that one, but the app fell into neglect and started lagging behind the competition in many aspects.
Edit: As it turns out, Antetype might be revived! [1]
