I think Oz was great, definitely groundbreaking in terms of what kind of content could be shown on television, but at times the series seemed very disjointed. Every season seemed to be build-up, build-up, big event in the finale, then cliffhanger. And when you'd come back the next season, certain storylines would be dropped, certain characters had disappeared, etc. Like they never knew if they were coming back, so they didn't plan for it. It made it difficult to get involved when you knew whatever storyline you had an interest in had the possibility of vanishing without even a throwaway explanation. But then again, this was more of a problem later in the show.
*MAJOR OZ SPOILERS*
The first season is excellent, eight of the best hours ever to hit the television screen. My only complaint is that they blew their load by having a prison riot in the finale of the first season. Imagine if the NY/NJ war happened at the end of the first season of the Sopranos. What could possibly be a bigger or more serious outcome of all the shit going on after a riot has already taken place? The prison blowing up? Oh yeah, they tried that...
Not to say that seasons two through the first half of four aren't great, I'd recommend to anyone that the first four seasons are required viewing. However, I believe the moment Adebisi was killed, the writers had no idea what to do with the show next. The second half of season four seemed to be spinning it's wheels, and it's also when they started introducing really goofy unbelievable shit to the story.
And that's the downfall of the show, in my opinion. Suddenly every member of the O'Reilly family is in the same prison? The best they could come up with for Dr. Nathan to lose her license was some anti-aging drug that turns one guy into an old man having a heart attack? Having murderers training seeing-eye dogs in the general population? Not to mention the fact that they decided to go supernatural at that point, which was such a tonal shift in some of the storylines that it felt like I was switching channels back and forth between two different shows set in a prison.
And the final season, outside one or two decent payoffs, was just a Mongolian clusterfuck of epic proportions. They chose the "Kill 'Em All" approach to wrap things up, which I've always felt is really lazy and cheap. In the space of eight hours, almost every major character is killed off, some in increasingly ridiculous ways, others as an afterthought. And don't even get me started on the fact that the show seemed to be in a rush to off every black person in the cast by the final episode. First Adebisi and Basil/Mobay, then Hughes, Hill, Said, Glynn, Omar...I'm trying to think if there was any black people left by the time the credits rolled.
*END MAJOR OZ SPOILERS*
So in conclusion, buy the first season and watch it, it's excellent. If you like it, which I'm sure you will, then get seasons two through four. Continue from there only if you're really curious, but if you're the type that gets let down when a good show starts falling apart, stop at four.