Gintama is a very unique (and odd) manga, but very captivating.

STORY:
Gintama doesn't really have a direction or course. It's like Doraemon, an adventure a day. But it is far more interesting. Sorachi-sensei gives hints of mysterious pasts, heavy under plots, and the like, but so far, he hasn't tackled fully any of those yet.
The basic format of this one is: everyday adventure, then exciting arc that gives some hint/quick peek of Gin's past or anything serious, then back to everyday adventure, and so on. Actually, this cycle would leave you wanting for more, and will make you crave for the next chapter.

ART:
regular shonen jump manga art. Not much toning, but it's still awesome. The fight scenes are understandable. There are a lot of close-up panels, which I find nice since it shows the expression/emotion clearly (plus, it adds to the effectiveness of the lines/words). One thing to point out, however, is that it is a bit wordy (too big speech bubbles), but you'll have to forgive Sorachi-sensei with this one, since Jump only gives 20 pages for a chapter. Either than that, it's good.

CHARACTER:
AMAZING and UNIQUE characters.
One thing that'll get you hooked into this manga is the presence of unusual (and weird) characters. I'm not talking appearance, but their personality. There is a kaboom of far-from-average people in this one.
However, despite them being not your ordinary characters, you'll still get attached with them as they have the human side that will make you sympathize with them.
One thing notable with Sorachi-sensei is his use of weird personalities to bring out the human in them. This is the first time I have ever seen this style before. Each and everyone of them has their own life, way of thinking, and principles that show what kind of soul they have (the main theme of the manga --as much as I can see).

ENJOYMENT:
This one I give Gintama 100% times infinity!
What's really captivating in this manga is the humor and the witty responses present. They're all unexpected (trust me.)
Sorachi-sensei has great humor. He uses a lot of puns on words (making a lot of translators whine) just as the title itself ("gintama" (gin's soul/ silver soul) almost like "kintama" (testicles)). With this, you can now deduce that some of the jokes toe the line a bit, and this makes Gintama a lot more interesting. Sorachi-sensei has a way with words, and his choice of words are really quite remarkable. (This explains the big speech bubbles).
Not once have I found a part corny or dull.

Overall, I'd totally recommend this one. I'm not a fan of light-plotted mangas or stories with no direction, but this totally is an exception.
You're not going to be after the story itself, you're going to be after the satisfaction it gives you while you read it. What you're going to look forward to is the script, the reaction, the turn of events, and the way this wacky world will turn out to be.

Thank you for reading my review, and I hope this gives help.

Oh, and one more advice:
if you're gonna watch the anime, READ THE MANGA FIRST!
There are a lot of edits in the anime for a lot of reasons, and it ruins the witty jokes of the manga, making it corny --believe me.