From what I've seen of the last four seasons, I'd say that it almost feels like seasons 1-3 are their own seperate arc compared to season 4. I do wish that there were more case-style episodes like we had in the earlier seasons that could tie in to the overall arc of the story.
Season 4 had a lot of emotional impact, which I think is good for the show as it helps the viewers relate to these characters and understand their side of this story.
Though, with the entire timeline shift I wonder what things would have been like if they hadn't changed. Would the Observers still invade? I do really wish that this 'Amber-verse' that we were introduced to in season 4 was another universe and that Peter had found his way back to the blue-verse and the original Olivia in season 4.
I'm going to miss the alternate universe too; I loved all those characters, and there's likely no more Lincoln

. I think it would have been SO cool if they could have kept the bridge open and had an allied force from both universes to combat the Observers in both universes. That would have been cool.
Does the show suck now? I wouldn't say so. There's still a lot that can be done with this final season, and I for one am looking forward to the Observer invasion/conflict and seeing the world's descent into the Orwellian-style universe we saw in "Letters of Transit".
Though, the one thing I think we can all agree on is that this is not the Fringe we were introduced to in 2008.