Google Drawings are convenient for their collaboration and ability to include in other Google docs, but I've mostly found that it seems optimized to produce ugly diagrams. It takes extra work to make things look lined up well.

I use it for the easy collaboration, but miss OmniGraffle every time I do.

You need good stencils for Google Draw. There a number of links out there for UML stuff and actually works quite well. Unfortunately, out of the box it’s definitely on the Spartan-side.