I do find these new episodes funny but nothing comparing to the hilarity of the first 5 seasons. The first seasons where beyond strange, funny and offensive on many levels, sadly the new epsidoes are more like social satire commentary.
I haven't really watched the older series, I started around two seasons ago. I think you're right in saying there isn't much re-watchability. Once you hear the big joke that an episode leads up to its brilliant for the moment but that's about it.
South Park episodes gives a good laugh every now then especially the one with Randys massive balls that was just hilarious. I liked maybe abit more when SP did its own episodes not taking everything from todays world but Im not judgeing, still funny.