I started watching Dexter shortly before season four aired. I'm up to date through season seven and won't get to see season eight until it hits Blu/dvd in November. Season two (Bay Harbor Butcher investigation) and season four (Trinity killer) are still my favorites, with season six (Doomsday killers) being the low point.
There have been some great supporting and guest characters on Dexter like Sgt. Doakes, Lila West, Arthur Mitchell, Agent Frank Lundy, and Miguel Prado. The last few seasons haven't had any such strong characters, though I did find season seven a big improvement over six.