I'd say start with S1. If you like the campy-ness of the show, then i think you can enjoy the others as i think the absurdity goes up a big notch each season. I've enjoyed all the seasons for the most part, but some parts of the "campy" have been pretty cheesy (esp S4). All in all, i've found it entertaining enough to keep up with the show.
I believe they have one more season, and to me, thats about all i think it should continue for.
@SheepTea - yea, the finale had some surprising elements in it. Not sure about the cliffhanger. I'd be willing to accept it if they go into a whole science spiel about the permutations of Hep V. But i don't know, i just don't want it to become the Walking Dead (no offence to TWD fans.....)