I'm facing the same issue as @Dennis123D above.
I'm able to export a sketch that contains splines by using the "Export to DXF (Splines as Polylines)" add-in. Then I run it through Kabeja to create an SVG file. (Direct import of the dxf file still leads to splines missing in Inkscape, which I don't quite understand why as they should already be converted to polylines.)
When I then import the svg file into Inkscape, it shows up very tiny, because nowhere in the svg file is it specified what the units are. And Inkscape is just using whatever "user units" instead of inches when importing the svg, even though my document is set up in inches.
Inkscape on my Mac seems to have a defined value of 90 pixels per inch, so a sketch of a 5x5 inch rectangle shows up as 0.056x0.056 inch (5/90) after import. In order to fix it, I need to scale everything by 9000%. My 5x5in rectangle then measures 5.01x5.01 inches in Inkscape. (The 0.01 error is probably due to rounding errors. When I print it out on my inkjet printer, the rectangle measures pretty much exactly 5 inches.)
In summary, my workflow currently is this:
1) Create part in Fusion 360
2) Export sketch as DXF file with "Splines as Polylines" add-in
3) Convert DXF file to SVG file with Kabeja (Using command line: java -jar launcher.jar input.dxf output.svg)
4) Import svg into Inkscape document (default units and rulers are set to inches)
5) Transform imported object (Shift-Ctrl-M): Scale 9000%
6) Clean up:
6.1) Stroke width is usually 0 after I scale the object
6.2) Remove stray lines. Some control handles for the splines get saved in the dxf file exported from Fusion. (Usually it's the handles I've given a constraint.)
I only started using Fusion360 last week, so I'm not sure whether there's an easier way of doing this. All I want is to place my design on an Inkscape document so I can send it to Ponoko for laser cutting (http://www.ponoko.com/starter-kits/inkscape). So far everything seems to be going great, even though the export step into Inkscape seems unnecessarily complicated.