I would say I am near obsessed with Billy and Mandy. This show is one of two, maybe three cartoons I ever turn straight to if it is on Cartoon Network at the time. I think the main reason I like it is because of Mandy; she makes Billy look like an even bigger idiot than he is, and makes the Grim Reaper look mediocre. She is smart, able to figure things out well, and her lines are especially funny due to her terse, deadpan attitude.
Grey DeLisle has got the best voice talents of anybody I've ever heard. She does such an outstanding job as Mandy, yet as Frankie Foster on Foster's Home for Imaginary Friends, she sounds like a different person; it's amazing. The other actors do well also, and they do a much better job than those on some Anime cartoons currently playing.
One thing I wish they would cut out, though, is a good portion of crude humor. I'm sure 90 percent of cartoons today have crudeness that should be omitted. However, it is a small price to pay for the joy of watching such an original cartoon that is by no means generic. Every episode is completely distinct, unlike some cartoons whose episodes all follow two or three plots in an entire series.