I downloaded and watched all 6 seasons of it (8 eps per season, 16 in season 4; don't ask, it's just that they decided to make season 4 a double season for some reason). It's the first of the HBO regular shows to hit a good level of popularity (Sex and the City, Six Feet and Sopranos obviously did better afterwards), so you can see it's a bit rough around the edges, especially to start. They were going for as much shock value as edgy, kinda of a way of saying "here's how it is; we're going to show it to you, so just deal with it". Now, I'm not complaining, but the shows that came after it were a little more polished.
Now, don't get me wrong, I really enjoyed it. The first three seasons are great, with season 3 just one great ep after another. The first half of season 4 was good, but it started to unravel a little bit after that. The show was always edgy, always making sure that you couldn't guess what's happening next. I won't tell you what happened, but you can't keep something like that going for too long without making some major changes (kinda like 24 did this year). Their transition occurred during the fourth season and the show lost a little bit of what made it great. So, the second half of season 4 it starts to slow down a little. Season 5 keeps going downhill; it definetely jumps the shark here, probably a few eps in. There are some good parts, but it starts to unravel. Season 6 is atrocious. Just don't watch it. It's a desecration the remainder of the show. They were so far past the shark at this point that it's almost like a copycat show that somehow had the same characters with the same name but wasn't the same show....
That's my take on it. HBO probably started focusing more and more on Sex and the City and Sopranos at about Season 4 and probably were working with Alan Ball on Six Feet, so you kinda get the sense they just let it go a bit. Too bad, I would have preferred the show to end on a high note, but definetely watch the first 4 seasons and if you really liked them, the 5th. If you just have to see what happened next (like me), check out Season 6, but it's optional.