Neat idea, but very broken. Too many bugs that destroy or damage mockups.
Very good, especially compared to UXPin.
• Excellent layer/group management. Clearly aiming at Photoshop in this regard.
• Stable: no crashes, and so far, only one weird little bug encountered (the order of elements in a group sometimes gets reversed, but fixes itself after putting the "last" element at the top, where it belongs.)
• Comparable commenting system to UXPin.
• Near feature-parity with Justinind.
• Very unstable (bugs, bugs, and more bugs: new ones every day!) I have had many hours' work destroyed by "smart elements" suddenly "exploding" all over the page. Copies of mock-ups lose grouping, making the groups useless.
• Because it's cloud-based, very sluggish: uploading a small icon graphic can take several *minutes* before the image is usable in a mockup.
• Terrible layers implementation: All but impossible to select items that are behind other items without moving the "upper" items out of the way. ZERO layer management. None.