Welcome to the club!
Michael C. Hall makes the show. Whether or not you like it will probably hinge on whether you find him personally riveting.
I loved S1, despite being thoroughly spoiled on everything. I think there's a vibe to S1 that they didn't successfully continue in subsequent seasons, mainly the notion that Dexter is a truly scary and dangerous person, who could be seduced to become a "bad" sort of serial killer. Instead, the show has been flirting with the notion that Dexter could in theory be reformed, which I think would be a betrayal of the show's premise. Also this season had more effective black humor. S2 also had some, but I've noticed it getting less and less, which is a shame.
S2 is my favorite of the series. Lila was a stupid character (she was obviously a plot contrivance more than a character, I hate seeing that) but the season-long conflict between Dexter and Doakes was great stuff, especially the scenes in the cabin.
S3 was the season when I realized Dexter
isn't always going to be awesome.
The notion of contrasting Dex with a killer who masquerades as an upstanding citizen was good, but the execution was off.
S4 was back to fine form. I wasn't spoiled on Rita's death, so that was a shocking scene. (I'd heard that the finale had a "big" death, and I was actually worried it could be Deb, or worse, one of the kids.) I thought Rita's death was a setup for a season when Dex really goes off the rails, something I still want to see. Of course when they take that step, there's no going back, so they'd need to be prepared to end the series within one or two seasons.
S5 - eh, didn't care for it. Dexter in full meltdown would have been far more gripping. Lumen was too much of a victim for this show. It might have worked if she'd become more of a psycho because of her experiences, perhaps even a danger to Dexter. Having Dexter be hunted by someone who he thinks is an ally, or possibly is falling in love with, would be an interesting plotline. The ending, with Lumen being "all better," was just bullshit. That's way too much of an easy out for a show like this. I'm really concerned that the writers are losing their nerve and pulling their punches.
S6 - I've decided that this show is no longer worth shelling out for Showtime, so it's back to Netflix DVDs for me. Maybe if S6 turns out to be back to form, I'll relent and start subscribing again, but they have to win me back. No more pulling punches. I also hope they aren't going to try to drag this story out so long that it loses its oomph and turns into a procedural.
I also was amazed to learn they shoot the show in LA. Much of the housing (and of course the beaches) looks very similar in both cities, but I had no idea there were wetlands/swampy areas around LA that could sub for Florida.