For the first season or so Gintama isn't really amazing, I mean it's alright and all with a few nice laughs but you can't truly appreciate Gintama until you've watched most of the characters. The jokes keep getting better later on adding in more over the top parodies (the fake ending ep.150) the gundam parodies and well... everything. Though the longest running joke that I love is "It's not Zura, it's Katsura!"
Have you watched Desert Punk? If you liked Issei I find it almost impossible to not like Desert Punk. They're pretty similar though Desert Punk takes it up a notch. And you haven't watched Trigun or Afro Samurai? At the very least you should know about Afro Samurai! I found out about Afro Samurai a long time ago... truly the most GAR black man to ever live. And did I mention he has the best afro?