Oh we love Tilt Brush because it is easier to use and display on virtual platforms like VRChat and Altspace, although rn those platforms are killing us with their 2018 to 2019 Unity update, so we have to rebuild our worlds in Unity 2019. Goro Fujita has a huge group that uses Quill and afaik they use it to render videos for the most part, I have seen Quill animations and meshes in VRChat by Super Rare, which is super rare because there is a lot of processing and clean up needed to optimize the mesh as with Tilt Brush too. Popularity-wise, well there's money in the Quill scene for animations so, there's that xD Some Tilt Brush pieces have been sold as NFT's for over 10K though, so there's that too.
Masterpiece, we still pop in there everyone once in a while but it is mostly used for the animation studio, but we prefer Tvori over MasterPieceStudio. Tvori is more popular than Quill in the VRArtLive group, but Fujit's group on FB has like thousands of people there, I dont follow the group too closely so Im not sure if that is just his fan base mostly or actual users.
Gravity Sketch, YES. That's our go to for designing clean meshes and we take those into Blender to optimize it (merge duplicate vertices like with Tilt Brush but only with the 3D brushes) and some use Substance Painter to paint on textures after. I think Gravity Sketch is the most popular and used tool out there other than Tilt Brush ofc, because Ford was/is using it with their concept artists.
Now we are all excited about Adobe picking up Medium and dipping into the VR/3D scene but sad that its an entirely new sub set to pay another $40 for.