I hate pretty much all diagramming tools. Visio is too expensive, Dia uses an ancient version of GTK+ and crashes when exporting, Calligra Flow crashes a lot and doesn't work on anything but KDE, LibreOffice Draw is pretty much the only one that isn't completely terrible but the canvas size is fixed and it defaults back to the selector tool after every operation. However, this whole HTML5 thing is finally starting to pay off.
Gliffy is a very non-terrible tool. In fact it's actually nice to use. It's written as a true Chrome App, ie: runs in its own window, writes to filesystem. This is as close to a desktop feeling web app as I've seen, controls are very responsive, and all the basic diagram shapes are built in. It supports drag-n-drop to get custom images into your diagram. You can associate your Google Drive as a cloud storage location or save to your local machine and use the data syncing option of your choice. Finally, the pricing model allows for a very reasonable try before you buy scenario.
I give it a thumbs up. Whatever that's worth.