The Slayers series for me is very interesting- I don't know exactly what made me start watching it but there is some hook that just pulls you in. I would reccomend though that most people maybe start with Slayers Next instead of this, mostly because unless you are a hardcore old-school fan, it takes a while to get into (especially since Crispin Freeman~, playing Zelgadis, doesn't come in for quite a while). By the second season (next) the show has definitely gotten much better, and it starts off fresh so you don't need to have seen the first season to enjoy it or follow the plot.
But enough with that, the plot. It has a very classic medieval-magic-and-swords theme, where you follow the characters as they travel across the land in search of... well, several things. I'm bad at explaining plots so I'm going to stop there. The series is a nice mix of adventure and humor, and often has filler episodes, although I wouldn't really call them filler, because most of the show is the characters getting into amusing situations during their quest.
It's very hard to explain, but if you don't mind how cliche it feels by todays standards, you should definitely go watch it.