In general, I'm quite impressed. The story does take some liberties with history, especially in the way it weaves our fictional protagonists Pullo and Varenus into the high-level political struggles of the day. On the other hand, it doesn't appear to violate the historical accounts too much: it just embroiders them with the myth of these two hidden but important figures, while tacitly treating some details of the usual accounts as later additions. The result is largely plausible, if sometimes ill-documented. It manages to make the political class of Rome look pretty sordid -- but then, so does Suetonius. Most of the characters are richly drawn, and most of them are allowed some real arc -- albeit necessarily a somewhat sketchy arc, since there are so many characters in this sprawling tale.
The cost of the production is reportedly what killed the show, and that shows up on the screen: it's lavishly done, and feels quite real to my admittedly layman's eye. The acting and direction are both delightful, if sometimes a bit scenery-chewing. The historical research seems good, with lots of fascinating odd details that have spawned a mini-industry of people trying to figure out what the *heck* is going on in the background. (Even the business of breast-feeding Caesar's corpse, which occasioned much head-scratching, seems to be at least somewhat justified by some academic examinations.)
I'm disappointed that it ended, but they chose a good place to stop -- the story comes up to the ascension of Augustus Caesar as the clear winner, which is when the actual history gets a lot quieter for a fair while, without the big famous battles that the first two seasons swirl around. It would have been fun to see what comes next (the series is mostly about everyone's personal lives, so I'm sure it would have made much of the court intrigues), but it would have been different. And the series ends well: while they left plenty of loose threads that could have been picked up if it had been renewed, the second season doesn't end with the sort of cliffhanger that the first does.
And I must say, it's fascinating to see Brutus get treated as practically the hero of the tale. In the entire series, I think he's the only person who comes across as consistently good, noble and well-meaning. (If a tad weak.) Sadly, he's on the wrong side of history, but they clearly made the conscious decision to mess with peoples' expectations by having him as simply an unfortunate man, trying to do the right thing but a little too easily manipulated by the stronger personalities around him.
Overall, great stuff -- I recommend seeing it if you have a chance. Note that you do want to get both seasons, though: they really form a single pretty coherent story, albeit one that sprawls across decades. (Be prepared for occasional jumps of several years between episodes...)