the negative reviews are because some people are getting confused with different storylines being introduced, how doomsday looks kinda like a ninja turtle, a lot of characters are being squeezed in the movie, and it doesnt have that "Marvel" humour.
I personally enjoyed the movie but i do agree it can be a bit of a headscratcher if you're not a DC fan or do not have a good knowledge of Justice League. This movie has the same dark feel as Watchmen so if you're used to Marvel-ish take on hero movies (comedic, feel-good superhero movie) then you will feel a little unsatisfied.
Affleck as batman was surprisingly great. He played the part well. In fact, i liked him better than Christian Bale's take on batman (flame suit on).
Fight scenes were great, some where a little over the top to the point where your eyes dont know what to follow.
Lots of easter eggs.