Ok, I completed Le Chevalier D'Eon and am ready to render my verdict.
On a scale of 1-10, I award it an 8.5/10 over all. This is a composite of many different areas, but on the pro side (and there are many, many pros), the art is magnificent (GONZO was in on it, along with Production I.G.), the music was wonderful, I liked both the opener and the closer equally, the story itself was very solid. It did get rather confusing near the end, but it wasn't an esoteric mess - it all made sense when it was done. The bonus historical notes were useful, especially if your knowledge of France or this time period was limited (mine wasn't, but there was still a good bit of information here that enlightened me). The story uses many historical characters, but plays with reality (it is fantasy after all), and time since some of the characters in the show died before their real-world time.
The folks that voiced the major characters in the English dub were great - D'Eon, Lia, Robin, Teillagory, and Durand were well done, as were many of the secondary characters - King Louis XV, Queen Marie, Ana, the Comte de Saint Germain, the Marquise de Pompadour (Shelley Calene-Black, the only voice actor I recognized by name), and Maximilien Robespierre. But this is where the cons begin... I can't say all the other secondary character voices were horrible, but many were. Cagliostro was terrible, as was the Russian (his name escapes me, but if they wanted to portray a deep voice, they should have gotten a voice actor with a real deep voice, not someone trying to grumble down to one), and the fruity little monkey at the French Embassy in London. Many others were terrible, but it was really the only significant blemish on this show - but for me it was a big one. After Cagliostro and the Ambassador were introduced, I had to switch over to Japanese again, I just couldn't stand them.
I don't know if I would buy, but it is a consideration, there are some other anime I would buy first, but it's worthy of some attention. Give it a go.