I’ve tried DIA http://live.gnome.org/Dia to make some network/deployment architectures graphics.
– I took me a while to understand how I add elements (in my case network elements) to the canvas.
– DIA works with the concept of layers, so if you want to put something in front of other object you must create a new layer, cut the object, change the layer and paste it.
– Linking objects works ok, but only on the same layer.
– After the drawing is made you can’t copy it and paste it on an OpenOffice document.
SO, as a standalone product, it’s good, despite the issue of layers depicted above. It means you must think your drawings a bit ahead, and if you want to make changes it can get quite complicated.