I put together a simple geo-converter a while back (and posted about it) that converts geojson to arcjson\esrijson. I needed it to get some polygons in an arcgis feature service. I have updated that converter to also convert from arcjson to geojson as well. I used it to pull arcjson from an ArcGIS Feature Service via a query against the REST API and convert the results to geojson.
The geoconverter tool
I pulled the arcjson formatted data from City of Fort Collins ArcGIS web services
A query against the Neighborhoods layer, like so:
Results into the geoconverter
Created a MapBox map with it