I've been using the Milling Cutters Store carbide slot drills of late. They cut mild steel in the small mill like butter.

Broken Slot Drill - Still the best-millingcutterstore-3flute-02.jpg

then I broke the cutting edge from one of the flutes

Broken Slot Drill - Still the best-millingcutterstore-3flute-03.jpg

rather than throw it away I carried on using it, not as good as a new bit, but better than any other slot drill I have.