I only ever watched the main series, because that is where the meat of things is, but yes, it really is that good.
Understand, the story covers over a half decade of in-story time (probably longer), a lot of the arcs are something like ten episodes long...
Legend of the Galactic Heroes is a story of politics, intrigue, heroism, war, and ideology, with events ranging from brilliant military stratagems and Machiavellian plots to philosophical discussions about the nature of government and society. It shows both the positives and negatives of both sides of the story (it starts out with Yang Wenli and Reinhardt Von Lohengramm as the protagonists, though things become a bit more varied late in the series), showing how an authoritarian regime can both bring horrid atrocities and glorious advancement and how even a democracy built on good intentions can be rotted from within by populism and military adventuring.