They are pure escapist fun - very crude and laugh-out-loud funny. Lots of action/violence, lots of sexual humor, lots of humor built around people's outrageous personalities. Not a lot of character development over the course of the long series (I've not read all, but enough of them).
They do start to seem all the same after a while, if you binge on them. I would not binge on them unless I were really sick, or depressed, or in some other kind of long, dark tunnel where I just needed a lot of dark laughs.
I wouldn't recommend buying all of them, since they are not the kind that you would go back and re-read (at least, I wouldn't). I think you can get them at libraries. Or, if you want to read them in order, you might want to buy the earlier ones (used?), if they are harder to find, then check the later ones out of libraries.
I think there are so many of them because they are sweet, addictive and disposable, sort of like Oreos for your brain.