Unlike the movies, in the TV show we get to see female immortals, immortals with glasses, immortals with spych and mental problems, and a secret society of mortals who watch what immortals do. We get to see how immortals have subtly and not-so-subtly affected the history and development of the human race.
Unfortunately, they didn't know when to stop. When the creativity was stale, when they lost their good sworc choreographer, they just kept going until they ground the show into the dirt.
Seasons 1-3 are pretty good. Season 4 isn't, and season 5 is horriblly awful beyond words. For some reason, the number of comedy episodes went from 0-1 to 50% or more. The quality of the swordfights went from decent-to-great to uninspiring-to-lackluster. The easy way to spot when they lose their swordmaster: all the fights are filmed with two characters wearing all black, against a black background, with minimal lighting, all so you can't actuallly see what's going on.
The Last episode of season 3 is called "Finale". I strongly recommend you stop there.