Dexter is one of those shows that once you get into it, you can't get out.
Season 1 is extremely gripping, setting the scene and tone for the rest of the series. Season 2 and 3 are interesting while you're watching, but nothing prepares you for the intense Season 4. Season 5 is probably the worst of the bunch, but when you're this far invested in the characters you still enjoy it.
If you've seen one or two of the seasons, or are thinking of getting into it, you can't go wrong buying all 5 in one go. Out of the recent hits; The Walking Dead, Game of Thrones and the classics like LOST and 24, Dexter is still the number one show I turn to for everything I could possibly want. Great acting, great cast just all around great.