Hi
There are a couple of issues with filter effects UI, that I'm concerned about in the upcoming release.
First, at some point of development, the blur slider was disabled when there was a more complex filter attached to the object. I believe that this is a good thing, because it's not too clear, what it should do.
Nevertheless, right now it causes a crash. I haven't dwelved much into this problem and there may be some bigger underlying problem, that needs fixing. But unless there's a plan on how this slider should work with complex filter already attached to the object, I believe it should be disabled in those cases.
(see https://bugs.launchpad.net/inkscape/+bug/186281 )
Second is a problem I fully realized only today: there's no user interface for setting enable-background property. Without this, the only working input images are Source Graphic and Source Alpha. Background Image and Background Alpha only contain data, when there's property 'enable-background:new' set on some parent object of the filtered object. Without this, they will just appear to not work.
(see https://bugs.launchpad.net/inkscape/+bug/189341 )
(Fill Paint and Stroke Paint don't work because there's no renderer support yet for them in Inkscape)