Firstly a bit of advice - if you buy the big boxset it is quite pricey. I would recommend buying them individually (if it works out cheaper) or trying the first season or try and catch one online to see if you like it.
(UPDATE!! - 12/12/11. Just noticed it's at 49.99 at the moment and if it stays at this, get the whole thing as that is a bargain price!)
(SECOND UPDATE! - AT £28, this is an absolute steal! Get it now!)
Before I'd watched 'Dexter' for the first time, I had the difficulty with the concept of it. I've always disagreed with an eye for an eye and even though I knew Dexter only killed 'bad' people that had killed others, I thought it sounded too weird to enjoy as a programme, let alone for 5+ seasons.
I don't know what I was thinking! 'Dexter' is absolutely incredible. It is by far the best thing I have seen on television in a long time. As I've mentioned above the show follows Dexter as a 'loveable' serial killer who has a 'dark passenger': his urge/need to kill. He is also a blood spatter analyst for the Miami police department which seriously makes for brilliant viewing. Each season is normally dedicated to a particular 'case' for the police department and there are numerous kills for Dexter that go alongside this but each episode keeps you gripped and often guessing.
There are some amazing characters and you follow Dexter's sister Deb throughout and his growing relationship with Rita from the first episode as he tries to balance his dark passenger with leading a normal life. You also follow the lives of the other Detectives in the Miami Met and their sub plots which just makes every episode interesting. Keep watching for a part played by John Lithgow in Season 4 - it is sheer genius.
I'm trying to get you to watch this and give it a go as I know without the push from a friend, I would never have tried it and now I am addicted. I'm currently eagerly awaiting Season 6 and I wish I were in your shoes about to start from the beginning. (UPDATE - Season 6 is incredible with some amazing plot twists and an ending that will blow your mind! - it's still far cheaper to buy this box set and then season 6 separately though!)
Surprisingly, Dexter really is loveable and he's got a great dry sense of humour which makes Dexter really quite funny at times. 'Dexter' is entertaining, tense, scary at times, even moving too. It has brilliantly written storylines played by superb characters - I just cannot fault it! Absolutely compelling viewing.