So far the bonus is pretty sweet. +28.7% wine productions (based on top of base productions) with a shrine of level 19 (out of 41) and blessing rate of 500 (out of 1000) is pretty spectaculair. My speculation is that the blessing rate stays active as long as the blessings period is active. And, yes, you read that right: the gods are already giving me more resources than the +20% production boost feature.
Total cost to bring shrine to maximum is about 22 million with best discounts. But only the last few levels are the most expensive resource burners, making it fair for smaller players still. But since you only need to build it once, it is a way better return on investment than your normal production boosters I think.
Things I've observed boosts the production blessings:
* the level of the shrine - achieved by building
* the blessing rate - achieved by donating daily reward points and keeping the blessing alive
* the building level of the shrine limits the impact, but so far it hasn't been a factor for me
Extending the blessing period:
* initial activation gives it a couple of days start bonus(?)
* adding blessings extends it a bit (seems more like hours than days though)
* for 600 blessing points, you can extend the period with 1 day for a god you pick
* for ambrosia, you can extend the period with 2/7/14 days - which is a massive benefit
You can assign the bonus to one city or all of them. So far I don't see a disadvantage for assigning one god to multiple ones, unlike what I've noted down earlier. Please correct me if you find something more specific. So far, I would say that the feature is pretty much worth it, so I'm going to be investing in expanding the shrine to maximum level first thing now.
Keep in mind there is a 20% production boost event this weekend, so the actual rewards of the gods will be 20% lower in normal situations. 
At shrine lv41, it costs 6m5 wood before reduction so it is really cheap to get there
