I found the first season to be great but feel it went downhill right after that as it lost the awesome tension that hung over everything in the first season (that first season just had a constant "tone" that perfectly fit, and I don't feel the later seasons had that). It gets downright tedious and typical mediocre drama a lot in later seasons.
The end of the series is so fucking dumb that I don't even know what to say. Its not "gunshot fade to black" level insulting, but its one of the worst endings I can recall. I think most people had checked out of the series by then though so not sure anyone really cared, but it was insultingly stupid like "hey thanks for sticking around, oh by the way fuck you." They should've had it end with him just getting hit by a fucking bus, would've been just as satisfying. Its a perfect example of a series that got padded out because it was popular and so they changed things to try and get more seasons. This could've been a really tight and well done like 2-3 seasons. If I remember things actually became contentious between the fans and the creators of the show (the fans whined about a lot of stuff, think it was Rita, then Stiles, then Colin Hanks; weirdly I was apparently the only person that took issue with how fucking stupid they handled things after season 1 by simply just ignoring it until they realized they should probably start finishing the series off).
Although, I think I'm in the minority in that I didn't find season 5 to be all that spectacular. Wait, why do people find season 5 to be so good? I remember people hating Stiles' character, and I guess I thought season 4 was season 5 and season 5 was season 6.
Something to think about: the brother and sister were fucking in real life for like half the series til he cheated on her with Julia Stiles. Granted they were adopted siblings (oh and also Deb realizes she wants to fuck Dexter at one point...).