Works well on: product photos against solid backgrounds, people against plain walls, simple graphics, logos. The segmentation is reliable for anything with clear subject-background separation.
Struggles with: fine hair, fur, transparent objects, low-contrast images where the subject blends into the background. These are the same limitations as most free tools.
The interface is dated but functional. Pixlr was designed before the modern design tool aesthetic, and it shows. This is not a problem for people who remember Photoshop 7, but it can feel clunky to newer users.