I'm currently playing through Disgaea 1 and I have 2 on standby.
I must say, Disgaea has to be my favourite Tactics game ever, for some reason I can't quite put my finger on. I personally found games such as Jagged Alliance and Final Fantasy Tactics (& War of the Lions) really boring, but this one stood out. Maybe it's the sense of humour or art style?
Anyway, if it helps yes I'd say its worth buying, and from what I've heard you can jump straight into 2 or 3 without playing the previous titles, as all you'll miss is the appearance of some familiar characters from the previous games' storylines.