I have been using this for years. This works extremely well for building and editing Chain physics for GMS, and I use it often, but it has some major issues users need to be aware of:
1. It doesn't properly support image scaling out of the box. Code it produces has to get rewritten like so:
physics_fixture_add_point(fix, xPos*image_xscale, yPos*image_yscale);
Where xPos and yPos are the numbers produced natively.
2. In addition, an offset dealing with scaling has to be used, like so:
var offsetx = abs(sprite_width/2-sprite_xoffset);
var offsety = abs(sprite_height/2-sprite_yoffset);
physics_fixture_bind_ext(fix, id, 0-offsetx, 0-offsety);
These two issues only come up if you're making a game with physics and are not strictly keeping pixel scales 1:1 (or are, say, mirroring objects in a Room with negative image_xscale)... but that's a lot of practical use cases, and I dearly wish this was fixed out of the box. Having to do this manually for something like a hundred objects at this point has been a serious time-suck.
3. Zooming in and out is a little crude and harder to work with than it probably should be. It would be nice if zoom scaling was an adjustable constant in an INI or something, so that different mouse-wheel settings or resolutions were better-supported.
4. It doesn't have any support for testing physics objects with GMS's new feature. GMS finally supports Box2D raycasting (takes small bow) but you cannot test raycasts directly in this right now.
5. This tool would be great if it had an auto-lozenge creator for pill-shaped physics bodies that slip past each other more readily would be handy, especially for newbies.
All that said... do I recommend buying this, if you're serious about developing for the GMS engine? Yes, I do.
None of the other Fixture editors explicitly support Chains, and Chains are really, really good for larger objects with complicated collision geometry. I just wish the author would come back to this project and either include the source code so we could build new features ourselves, get in touch with me about letting me fork it, or take up the suggestions shown above to explicitly handle object scaling during runtime.