Well, I did love season one, so watching the second season was worth it for me. I'd have to poke around (more than I have time for at the moment) to recall what each season's story well enough to remember which ones I loved the most besides the first, because my liking for them has varied. Definitely found the season with John Lithgow compelling, and definitely found the most recent two seasons lackluster--I mean, I enjoyed them well enough for lazy watching, but I didn't get that sense of "must watch next episode now! (Even though, since I'd already set aside the time for my binge, I did in fact watch the next episode within minutes of finishing the prior one.)
I did read the first book of the series, don't remember much about the potentially gory details; just remember that I preferred the show--at least partly because in the show Dexter seems more, well, likeable (which is key to having the show survive on tv for so many seasons, and may not be so important for a book). Felt no desire at all to read additional books in the series.