I have watched the first two episodes so far and it isn't bad. I was expecting it to be a lot more 'gore' oriented when instead it actually reminds me a bit of David Lynch's adaption of Dune.
Based on the movie, the series takes place about 25 years after angels, led by Gabriel, have nearly exterminated mankind. The war never really ended as much as the losses on both sides left so few survivors that they have more or less secluded themselves into their own parts of a devastated Earth. Humans, aided by the angel Michael, are holed up in mega-cities that are the last pockets of civilization. The mega-cities are ruled by "Houses" that are separated by family lines. There are houses based on military, religion, and even gender (all women). The Houses have become complacent and spend time working as much as against each other as they do the angels. With the factor of the angels involved there is a thin-line between politics and religion to the point where the two nearly become one & the same. A 'Chosen One' is destined to lead & unite mankind in the war that is about to start back up again.
The reason for the angels anger is the same as we've see in other works -- Gabriel and others are angry at their 'father' because he has chosen his creation, humans, over them and for that they vow to destroy humans until the day their father returns so the gates to the holy kingdom are opened up to them. It is heavy stuff at times. Odd as it may sound, watching the movie Dogma would act as a nice introduction to the mythology before watching Dominion.
There are some risque scenes and SyFy gives plenty of warnings that the show contains nudity, violence, and mature themes.
If you are looking for a straight horror series with blood & gore then you might not like Dominion. By contrast if you like dark dramas with a bit of intrigue then give it a shot.