Gintama is probably the most hilarious anime I've ever seen.
There really isn't a great story to speak of, you kinda go throught a number of advetures some take one episode other take 15. sometimes the you'll laugh so hard your stomach hurts and at other times Gintama has really good fights.
one good tip if y wanna watch gintama, make sure you've watched other popular anime cause a lot of references to other shows like hunter x hunter (or star wars :P) will be made in gintama.
The animation isn't really the best I've ever seen but it really fits gintama as it's supposed to play out in late 1800 so it has to fit the "mood". There is a wide variety of sound effects/music because there are just so many different situation that all require different sounds :P
the characters are all great, in the beginning there are just a few but it starts to build up to a big cast in no time at all. Every character get a good introduction and some time to show off, and everyone makes appearances throughout the show so there's nobody (important) you only see in one episode.
So if y wanna laught you're ass off, I say: "go watch gintama!" :D