Progrid Omni 8 is a departure from the 7 version in that the 7 had both moderate and ultimate styles depending on how much stability you wanted. The Progrid Omni 8 is closer to the 7 moderate version in medial EVA firmness as shown below.
The other big difference is sizing. Progrid Omni 8 runs 1/2 size shorter than Progrid Omni 7. Therefore, if you’re used to wearing a size 10 in the Omni 7 then you will want to size up to a 10 1/2 in the Omni 8. Below is a comparison length between Progrid Omni 8 W and Asics GT-2140, both in women’s size 8. The 2140 is true to size whereas the Omni 8 is not.
Dr. Jenny Sanders Shoe Blog 
I just bought a pair of these… They’ve been pretty good so far. I’m used to having nike’s and usually I wear a 7 in those. But with these I got a 7.5 because the 7s were actually to small. The person helping me wasn’t actually that helpful, to tell the truth. I mean, I’ve read that these shoes are for people with flat feet who pronate. I have normal arches, and I tend to underpronate, or supinate. Whatever, these’ll work out well hopefully.
Nike often run small. If you haven’t worn Nike before or you are switching to another brand, you will always want to double check the size.
-Dr. Shoe