I stumbled upon this when I about to draw a wireframe for a new project on paper, so I decided to give it a try. Here's my comments, as I'm creating the mockup:
- it's usable. Probably easier than Photohshop to throw together something quick and simple
- There's no way it's going to replace Photoshop anytime soon since I need a many more tools to get a real design done but it works for mockups.
- I don't get why the height is limited. My canvas is very wide (24 inch display) but only about 50% of the available space. Already found out from the comments how to resize
- it's hard to position elements exactly: it behaves differently from Photoshop near the guiding lines, which takes time to adjust to
- can't do ctrl-c, ctrl-v to duplicate objects. Again, it's a pattern I'm used to
- when the number of objects becomes large and overlapping, the list of layers will be useful probably. Or, I'll move to photoshop for anything complicated anyway
- when I hold the shift key down, I expect the element to move much quicker (I guess Photoshop does it this way)
- I cannot make only part of the text underlined. If I have a piece of text with some links in it, I'm forced to create individual elements for all links