It doesn't come off a Grand as an old Hollywood Epic, or even as grand as I, Claudius (i'll be comparing to I, Claudius a lot here). But the sets look decent, it's in Black and White which it utilizes well, and it more or less doesn't use a score which like in Dracula(1931) can give it an interesting vibe.
For Historical accuracy, some of it depends on how you interpret the evidence, but I think it's better then most including I, Claudius. I liked seeing all three of Caligula's sister acknowledged for a change.
The key to why it's so good is the Acting, all the performances are Great, it's done with a Shakespearean/Shaw style, the dialogue is all well written and the veteran actors deliver them all well. It' highly entertaining and re-watchable.
The writers must have been familiar with the novel of I, Claudius, because they mostly followed it's characterization for Claudius (Freddie Jones), and he does it quite well.
The most prominent performance is Tiberius (Andre Morell) who is the real star for the first 13 out of 18 episodes. It's a very unique and interesting interpretation of the character, and the performance is quite intriguing.
Ralph Bates as Caligula is a very good performance, beats John Hurt and Malcolm McDowell easily. I disagree with making him so villainous already before the illness, but at least he's not born Insane in this take. And I also don't like them making him kill Drusilla, and interpreting his other Sisters conspiracy with Lepidus as being made up.
Other notable performances are Barrie Ingham as Sejanus (Though here I do prefer Patrick Steward) Donald Eccles as Nerva, Suzan Farmer is quite alluring as Livilla. For some reason IMDB doesn't list who played Piso, he was interesting, and Eric Flynn as Germanicus was as well, they played off each other nicely (I never liked how I, Claudius skipped that whole phase and went right to the trial).
But my favorite performance was Caroline Blakiston (Mon Mothma from Star Wars Episode VI: The Return of The Jedi) as Agrippina. She was strong-willed, ambitious, stubborn but also noble. And I find myself very attracted to her even though she's not trying to be sexy at all. It's by far the most powerful performance. Especially after Germanicus dies, her drive to avenge him is captured well. It's a truly captivating performance, and exactly how I envision the character of Agrippina The Elder. (The Younger I envision as trying to be the same as her mother, but while possibly more intelligent and patient, lacks her noble virtue and honor, this miniseries didn't carry the story far enough to show that however).
So in my view if your a fan of history, or of good theatrical acting, I highly recommend it.