Lots and lots of fun. I'll break it down.
Amazing makeup and fantastic costumes (most of which are surprisingly accurate from a historical perspective).
Creative plot that manages to keep you interested through the first half of the season and absolutely hooked through the second half (some serious expense; it keeps you guessing without being random)
Good score (though it is a sin on their part not to include any Celtic music).
Generally brilliant acting (even the actors who struggle in the beginning literally get their act together; and the ones who do well, do REALLY well).
Not actually a rip off of GOT, calm yourself. After two episodes this is 100% clear.
Crazy good cinematography, editing, and set design. Stunning.
Some funny scenes, but nothing that seems entirely forced (maybe with the exception of the last part of the Brutus and Philo storyline)
Very, very binge-able.
Decent action sequences (not too many and not poorly done, just nothing too exceptional).
For my fellow history nerds: by the low, low standards of modern television, this show is pretty good when it comes to historical accuracy (luckily enough there aren't TOO many accurate accounts form the time) and the show runners are not guilty of the greatest of sins: transposing modern morality and modern ideologies into something that happened ca. 2000 yrs ago. No hardcore politically fueled revisionism, just creative license and mucking about.
I haven't had this much fun watching a TV show in a while, didn't feel tedious and I didn't get bored. Quite the ride really and, yes, I watched the whole season before commenting.