Considered an anime classic, and rightfully so, Trigun is everything you could hope for in a series. If you want to get into anime but don't know where to start, then I highly reccomend Trigun. It's a wonderful introduction to the world of anime.
The storyline is, for the first two-thirds or so, episodic but in a way that still allows the characters to move forward and develop- which I think is the main reason for its frequent comparisons to Cowboy Bebop which employs a similar technique. However, around about episode 17, the plot twists in a way that I doubt anyone would see coming judging by the previous episodes. It becomes a wonderful flowing arc from then on, as the characters you've grown to love in their episodic adventures are challenged and changed.
The clearest flaw of Trigun is in the animation which is truly showing its age. Action sequences are great, don't get me wrong, but the overuse of still frames (or any use, really...) with sound effects or even whole scenes of dialogue playing over it really bothers me. For me, it takes me out of the Trigun universe for a brief moment when I want to absorbed. But this is only minor, and the less-than-perfect animation barely detracts from the overall quality at all.
Not much to say about sound except that the music is totally, epically awesome. Again, like in Cowboy Bebop, the music fits in with every scene it plays and heightens the atmosphere enormously. I haven't watched it in Japanese with subtitles, but the English voice actors do a great job at realising their characters and putting forth all emotions.
Vash the Stampede is one hell of a character. He is completely loveable right from the start. With a friendly personality, peace-loving ways and brilliant laugh, he's just an all-round great guy. But the whole time, the audience has a sneaking suspicion that he is holding a dark secret from his past- and they're be right. With complex emotions and actions, Vash is a fantastic character. Those supporting him in the series are nothing to be scoffed at, either. Meryl, Milly and Wolfwood who can't help but follow him around or running into him over and over again, form strange and interesting relationships with him and each other. Even the main baddies Legato and Knives are written with depth and finesse. As I always say, it is the characters that make a good series great, and Trigun is proof of that.
Maybe I'm biased in giving Trigun a 10/10. It is by no means a flawless series. But, for me, Trigun will always hold a special place, not only because it introduced me to the world of anime beyond Pokemon and Sailor Moon, but also because it was probably the first show I saw where the characters and storyline compelled me so completely that I forgot the world at times. So, in my opinion, that is what drives me to give Trigun a perfect score.