First of all, I am a die-hard fan of Dexter and CAN'T WAIT for the seasons 1 & 2 to be FINALLY released on blu-ray (what are they waiting for ? especially since the US blu-rays are zone-protected).
But starting with season 3, the quality of this show has started to go down ("dexter's got a friend", "dexter's got a baby", etc. the character feels like a teenager, goofing around all the time). Not only the overall quality of art direction has started to become below average (where it was really outstanding in seasons 1 & 2) but the overall storylines are uninspired at best, and totally boring and ridiculous most of the time (Rita who keep spending her time yelling at Dexter as if he was a 3 year old child, cheesy dialogs, etc.).
Don't expect to get the same thrills and tensions you had with the "ice truck killer" in S01 or "Doake's hunt" in S02.
Season 4 was a little bit better than the season 3, but not by much (and it is DEFINITELY not the best season of this show). Having seen Season 5, I would recommend it as it has gotten at least bearable to watch, but again, it is nowhere near the 2 first seasons.