Well, this topic is old but there's no traffic on this area so I guess no one cares if I comment. I just read the whole series over a couple of weeks for the first time, so it's interesting to think about. This will be full of spoilers since I'm assuming you've read everything to view this topic.
Yes, I think it's hard to not notice the huge difference in the story before Griffith becomes a God Hand and after. The manga is amazing in that not too far in we learn of the God Hands and that someone Guts once new will become one of them. It's even more amazing that then we go into the backstory of what happened. So you're reading the whole time, about these extremely complex people with their own goals and humanity even if they're what we think of as evil, like Griffith, and you know that it's a forgone conclusion that all ends in ultimate betrayal and horror. That in itself is amazing. I don't know Miura Kentaro realized how great he hooked everyone or not by revealing the end ahead of time, but it works brilliantly. There's never a dull moment in that part of the series because you analyze everything for signs of the coming doom. Also the tension between Caska and Guts is amazing. Their whole love story arc is just ridiculously believable. When they finally get together it's because they are only people for each other in the world. You know there will be problems, but they promise to love each other, and it makes sense, even with Guts still disturbed about his own rape and Caska still partially conflicted about Griffith.
I just found that the whole of that half of the series was brilliant. You never felt like anything was reaching for a new plot, there was enough plot bubbling out of the character's psyche's.
As far as after the first eclipse, well I admit I started to get confused. The event itself is a mixed-bag. Griffith is awfully quick to listen to some frickin DEMONS about what he should do with himself. He's committed some political dirty deeds, but he's actually done a lot of heroic things up until that point. Suddenly, he not only listens to strange monsters, sacrifices his loyal men, but he rapes Caska, which is almost absurd. I guess it's probably not much of a stretch to say Griffith is a repressed homosexual and wanted to destroy Guts for loving her, but again things really start to fall apart here in terms of us actually knowing that and just being left to guess. Griffith's sudden turn to the "dark side" for power is still basically a mystery other then that he really liked the idea of being king of a country. Is that enough to rape and kill everyone? What scarred his psyche to that point?
After this, the story is lacking in a footing. Guts has to wander about in the darkness trying to figure things out. Caska has been turned into a pathetic, mentally-ill lunatic, to the point of drooling on her himself, wandering off, and being a sad excuse for what was once the only women strong enough to be a leader in Griffith's army and the lover of a mad superhuman like Guts. Before she had her own dilemmas and depth, now she's... nothing... which I think for a lot of people was a disappointing direction for her. Maybe you can argue it's a set-up, we're having to put up with Caska being this disgraced for some eventual turn around where she gains a whole lot of power from it.
The only new prophecy is the king of the elf land will be able to cure her but it won't be what Guts is thinking. That's not really a motivating thing to read for. Oh boy, she'll get her mind back but something else will suck! I sort of want a new cycle to get ordained by now, like we knew the original eclipse was coming. It's more fun when you know that everything is contributed to some important fate. Right now, the whole fate of everything is a mystery, even including what the hell Griffith's life as a God Hand was and how that suddenly jumped to the whole Rebirth thing. It's a series with so many scopes, that it would be good if Miura Kentaro prepared us for the next center point of the epic rather than just leaving us wondering.
Overall, the series is incredible though. My gripes about the second half are more just because it's a bit different and requires more patience and preys on the reader more by not letting him in on what things mean. It's still quite good, and if I hadn't read the first half, would be quite good. And really isn't the only true downer how long new chapters are taking?