I have only seen commercials for it - what mad it so bad?
I can give you my moron perspective.
Disclaimers: I'm not a comic book guy so I can't comment on how this matches up with any source material. I typically like most movies. I give a lot of latitude to most entertainment and just try to enjoy the ride. And possible spoilers ahead.
As I mentioned it took me three sittings to get through it. This isn't necessarily a criticism of it as I started watching it later at night and was tired, but a good movie will usually keep me awake. The first 30-40 minutes of this one did not. I found myself nodding off. The major problem I had with this first segment was how derivative all their powers seemed. Each time a new power was introduced I couldn't help thinking "oh, the same power as 5 other superheroes." There was even a point where I began wondering if this was Marvel blatantly ripping off DC. Ikarus = Supes, Thena = WW, etc.
I watched another 30ish minutes the next night and again found myself nodding off, but turned it off right after Sersi found out about the grand hidden plan so I was at least hooked to finish and see how it resolved.
Finished it the 3rd night, and at least this last part had enough to keep me awake to the end. I was hoping for a more interesting resolution but didn't get it. Just pretty meh. Overall there was just very little about it that was good. I can't think of any fight scene that was actually exciting. The resolution was lazy. I think there were missed opportunities with the Deviants.
Two subplots I actually liked: Gilgamesh and Sprite. Although neither subplot was entirely original, I still found them compelling. I also thought the Kingo/Karun comic relief was decent.