Great concept and smart execution. A suggestion - rather than rating "A" through "E" why not change to the more recognizable (for US audience at least) scale of "A through F" (A/B/C/D/F) which we're all mercilessly trained to recognize through years of school grades? "E" as your worst rating confused me at first glance - could be interpreted as "Excellent"
A-E is logical I'd say.