What do you like best about Draw.io?
I've used quite a few vector diagramming tools, but I keep coming back to this one as once you get used to its interface, it's both powerful and quick to use.
It provides all the usual vector drawing features, such as sticky connections between items, straight or curved connectors, attaching text to the inside of items, raising and lowering items, grouping, etc. It also provides a good selection of templates with some advanced layout features such as tables inside UML class entries, etc. I regard these all as essential, yet some basic editors still force you to, say, drag objects and their associated text separately.
In addition it allows you to build a custom library of grouped objects using a simple drag-and-drop mechanic, so you can save commonly used groups or styled objects. it supports many styles such as gradient and hatched fills. It has flexible connectors which can be curved or straight with either square or rounded corners, and it can even introduce "jumps" where connectors cross each other to disambiguate what connects with what. Text can be in any font on Google Fonts and has many flexlble layout options. Objects can be arbitrarily rotated and flipped as needed.
As well as the vector drawing features, the application can be run online or can be downloaded for full offline operation on Windows, MacOS or Linux. When you install locally, you can configure custom styles, colours and fonts so that they're always available.
Finally, the killer feature for me is that when exporting as PNG or SVG (or other formats), you can attach the original source diagram as metadata within the file. This means that you don't need to, say, export as SVG for a website and then also save a separate source file—you can check one file into your repository and just re-open it within the editor to make further changes.
All in all, I've never found any editor which offers a better feature set. It might take a little while to learn to use the more advantaged features, but it's well worth it! Review collected by and hosted on G2.com.
What do you dislike about Draw.io?
There are some very minor niggles with the tools. The sheer number of features comes with a cost in terms of ease of use, although basic features are very straightforward to use—just drag and drop shapes into your canvas, and double-click to edit.
Sometimes when shapes are very small it can be difficult to move them, because the drag handles for resizing have quite a large active area. However, this is easily mitigated by zooming in to deal with them.
Editing some objects which use the table features, such as UML classes, can be baffling until you find some of the more obscure options hidden away on the "Arrangement" tab—indeed, the fact that the right-hand properties bar has tabs at all can be a little confusing to first-time users. However, it's hard to say how the developers could have presented the sheet amount of functionality without making some things less immediately obvious to the user.
There is no concurrent editing capability that I'm aware of, so it's not as useful as a shared whiteboard, but as a single-user diagramming tool it's fantastic. Review collected by and hosted on G2.com.